fix(config): validate ContactEmail and seed it conditionally

The tenant contact address had no server-side validation, so a direct API
caller could store any string in the field service notices are sent to.
UpdateTenantConfiguration now rejects non-addresses, recipient lists (a comma
would silently widen who receives those notices), display-name forms and
control characters (mail-header injection), and normalizes what it stores.
Empty stays valid - having no contact address is a legitimate state.

The seed path had a worse problem than missing validation. It was a
read-modify-write of the whole model through SaveTenantConfigurationAsync,
which upserts with TableUpdateMode.Replace and no ETag: a concurrent portal
save was clobbered by the seeder's stale snapshot of EVERY field, not just
ContactEmail. It also wrote straight through IConfigRepository, bypassing the
TenantConfigurationService cache, so a seeded address stayed invisible for the
full 5-minute TTL.

TrySeedTenantContactEmailAsync now writes ETag-conditionally in Merge mode -
one property, and a 412 means someone else wrote the row so the seed abandons
rather than overwriting the winner. A lost seed is recoverable on the next
maintenance run; a lost admin edit is not.

The same seed logic existed twice (preview notification-email save and the
maintenance backfill). Both now call a single owner on
TenantConfigurationService, which holds the never-overwrite invariant, the
cache eviction and the fail-soft guarantee. Fail-soft lives in the service
rather than the repository so it holds for any implementation.

Tests cover the validator, the service contract, and the repository against a
mocked TableClient: Merge mode, the ETag actually passed, only ContactEmail in
the patch, the 412 race, 404, storage faults. Plus a regression pin that
SaveTenantConfigurationAsync is never called from the seed path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions.Tests/AuthFunctionSideEffectTests.cs

@@ -55,7 +55,7 @@ public class AuthFunctionSideEffectTests
         { CallBase = false };
 
         var previewMock = new Mock<PreviewWhitelistService>(
-            configRepo, cache, Mock.Of<ILogger<PreviewWhitelistService>>())
+            configRepo, cache, Mock.Of<ILogger<PreviewWhitelistService>>(), _tenantConfigMock.Object)
         { CallBase = false };
 
         _telegramMock = new Mock<TelegramNotificationService>(

src/Backend/AutopilotMonitor.Functions.Tests/TenantContactEmailTests.cs

@@ -1,4 +1,19 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Azure;
+using Azure.Data.Tables;
+using AutopilotMonitor.Functions.DataAccess.TableStorage;
+using AutopilotMonitor.Functions.Functions.Config;
+using AutopilotMonitor.Functions.Services;
+using AutopilotMonitor.Shared;
+using AutopilotMonitor.Shared.DataAccess;
 using AutopilotMonitor.Shared.Models;
+using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Moq;
 using Xunit;
 
 namespace AutopilotMonitor.Functions.Tests;
@@ -9,6 +24,8 @@ namespace AutopilotMonitor.Functions.Tests;
 /// </summary>
 public class TenantContactEmailTests
 {
+    private const string TenantId = "77777777-7777-7777-7777-777777777777";
+
     [Fact]
     public void ContactEmail_defaults_to_null_so_absence_is_distinguishable_from_empty()
     {
@@ -22,4 +39,348 @@ public class TenantContactEmailTests
         var config = new TenantConfiguration { ContactEmail = "it-operations@contoso.com" };
         Assert.Equal("it-operations@contoso.com", config.ContactEmail);
     }
+
+    // ------------------------------------------------------------------
+    // Server-side validation (UpdateTenantConfigurationFunction)
+    // ------------------------------------------------------------------
+
+    [Theory]
+    [InlineData("it-operations@contoso.com")]
+    [InlineData("first.last+autopilot@sub.fabrikam.co.uk")]
+    [InlineData("UPPER@CONTOSO.COM")]
+    public void ValidateContactEmail_accepts_real_addresses(string email)
+        => Assert.Null(UpdateTenantConfigurationFunction.ValidateContactEmail(email));
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void ValidateContactEmail_treats_absence_as_valid(string? email)
+    {
+        // No contact address is a legitimate state — it means we cannot reach this tenant.
+        Assert.Null(UpdateTenantConfigurationFunction.ValidateContactEmail(email));
+    }
+
+    [Theory]
+    [InlineData("not-an-address", "single")]              // no @ at all
+    [InlineData("@contoso.com", "single")]                // empty local part
+    [InlineData("ops@", "single")]                        // empty domain
+    [InlineData("ops@@contoso.com", "single")]            // two @
+    [InlineData("ops@localhost", "dotted")]               // unreachable bare host
+    [InlineData("ops@.contoso.com", "dotted")]            // leading dot
+    [InlineData("ops@contoso.", "dotted")]                // trailing dot
+    [InlineData("ops@contoso.com, attacker@evil.test", "single address")] // recipient list
+    [InlineData("ops@contoso.com; attacker@evil.test", "single address")]
+    [InlineData("Ops Team <ops@contoso.com>", "single address")]          // display-name form
+    public void ValidateContactEmail_rejects_values_that_are_not_a_single_address(string email, string expectedHint)
+    {
+        var error = UpdateTenantConfigurationFunction.ValidateContactEmail(email);
+        Assert.NotNull(error);
+        Assert.Contains(expectedHint, error!, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void ValidateContactEmail_rejects_header_injection()
+    {
+        // Once this address is actually mailed, an embedded CR/LF would forge mail headers.
+        var error = UpdateTenantConfigurationFunction.ValidateContactEmail(
+            $"ops@contoso.com{(char)13}{(char)10}Bcc: attacker@evil.test");
+
+        Assert.NotNull(error);
+    }
+
+    [Fact]
+    public void ValidateContactEmail_rejects_an_over_long_value()
+    {
+        var local = new string('a', UpdateTenantConfigurationFunction.MaxContactEmailLength);
+        var error = UpdateTenantConfigurationFunction.ValidateContactEmail($"{local}@contoso.com");
+
+        Assert.NotNull(error);
+        Assert.Contains("at most", error!);
+    }
+
+    // ------------------------------------------------------------------
+    // Seed contract (TenantConfigurationService — the single owner)
+    // ------------------------------------------------------------------
+
+    private static (TenantConfigurationService service, Mock<IConfigRepository> repo, MemoryCache cache) BuildService()
+    {
+        var repo = new Mock<IConfigRepository>();
+        var cache = new MemoryCache(new MemoryCacheOptions());
+        var service = new TenantConfigurationService(
+            repo.Object, NullLogger<TenantConfigurationService>.Instance, cache);
+        return (service, repo, cache);
+    }
+
+    [Fact]
+    public async Task TrySeedContactEmail_when_the_write_lands_reports_success_and_drops_the_cached_row()
+    {
+        var (service, repo, cache) = BuildService();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com")).ReturnsAsync(true);
+        cache.Set($"tenant-config:{TenantId}", new TenantConfiguration { TenantId = TenantId });
+
+        Assert.True(await service.TrySeedContactEmailAsync(TenantId, "ops@contoso.com"));
+
+        // The repository writes behind this cache; without the eviction the freshly seeded
+        // address would stay invisible for the full 5-minute TTL.
+        Assert.False(cache.TryGetValue($"tenant-config:{TenantId}", out _));
+    }
+
+    [Fact]
+    public async Task TrySeedContactEmail_when_the_tenant_already_owns_an_address_leaves_the_cache_alone()
+    {
+        var (service, repo, cache) = BuildService();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()))
+            .ReturnsAsync(false);
+        var cached = new TenantConfiguration { TenantId = TenantId, ContactEmail = "owner@contoso.com" };
+        cache.Set($"tenant-config:{TenantId}", cached);
+
+        Assert.False(await service.TrySeedContactEmailAsync(TenantId, "ops@contoso.com"));
+
+        Assert.True(cache.TryGetValue($"tenant-config:{TenantId}", out TenantConfiguration? still));
+        Assert.Equal("owner@contoso.com", still!.ContactEmail);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public async Task TrySeedContactEmail_with_no_address_never_reaches_storage(string? email)
+    {
+        var (service, repo, _) = BuildService();
+
+        Assert.False(await service.TrySeedContactEmailAsync(TenantId, email));
+
+        repo.Verify(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()), Times.Never);
+    }
+
+    [Fact]
+    public async Task TrySeedContactEmail_with_no_tenant_never_reaches_storage()
+    {
+        var (service, repo, _) = BuildService();
+
+        Assert.False(await service.TrySeedContactEmailAsync("  ", "ops@contoso.com"));
+
+        repo.Verify(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()), Times.Never);
+    }
+
+    [Fact]
+    public async Task TrySeedContactEmail_trims_before_it_reaches_storage()
+    {
+        var (service, repo, _) = BuildService();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com")).ReturnsAsync(true);
+
+        Assert.True(await service.TrySeedContactEmailAsync(TenantId, "  ops@contoso.com  "));
+
+        repo.Verify(r => r.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"), Times.Once);
+    }
+
+    [Fact]
+    public async Task TrySeedContactEmail_never_writes_the_whole_configuration()
+    {
+        // Regression guard: the seed used to be a read-modify-write of the full model through
+        // SaveTenantConfigurationAsync, which replaces the row unconditionally — a concurrent
+        // portal save was clobbered by the seeder's stale snapshot of EVERY field.
+        var (service, repo, _) = BuildService();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()))
+            .ReturnsAsync(true);
+
+        await service.TrySeedContactEmailAsync(TenantId, "ops@contoso.com");
+
+        repo.Verify(r => r.SaveTenantConfigurationAsync(It.IsAny<TenantConfiguration>()), Times.Never);
+    }
+
+    // ------------------------------------------------------------------
+    // Seed trigger (PreviewWhitelistService)
+    // ------------------------------------------------------------------
+
+    private static (PreviewWhitelistService preview, Mock<IConfigRepository> repo) BuildPreview()
+    {
+        var repo = new Mock<IConfigRepository>();
+        var cache = new MemoryCache(new MemoryCacheOptions());
+        var tenantConfig = new TenantConfigurationService(
+            repo.Object, NullLogger<TenantConfigurationService>.Instance, cache);
+        var preview = new PreviewWhitelistService(
+            repo.Object, cache, NullLogger<PreviewWhitelistService>.Instance, tenantConfig);
+        return (preview, repo);
+    }
+
+    [Fact]
+    public async Task SaveNotificationEmail_persists_the_address_and_seeds_the_contact_field()
+    {
+        var (preview, repo) = BuildPreview();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com")).ReturnsAsync(true);
+
+        await preview.SaveNotificationEmailAsync(TenantId, "ops@contoso.com");
+
+        repo.Verify(r => r.SaveNotificationEmailAsync(TenantId, "ops@contoso.com"), Times.Once);
+        repo.Verify(r => r.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"), Times.Once);
+    }
+
+    [Fact]
+    public async Task SaveNotificationEmail_clearing_the_address_does_not_seed()
+    {
+        var (preview, repo) = BuildPreview();
+
+        await preview.SaveNotificationEmailAsync(TenantId, null);
+
+        repo.Verify(r => r.SaveNotificationEmailAsync(TenantId, null), Times.Once);
+        repo.Verify(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()), Times.Never);
+    }
+
+    [Fact]
+    public async Task SaveNotificationEmail_still_succeeds_when_the_seed_blows_up()
+    {
+        // The notification email has already been persisted at that point; a failing side
+        // effect must not turn a completed write into a failed request.
+        var (preview, repo) = BuildPreview();
+        repo.Setup(r => r.TrySeedTenantContactEmailAsync(It.IsAny<string>(), It.IsAny<string>()))
+            .ThrowsAsync(new RequestFailedException(503, "throttled"));
+
+        await preview.SaveNotificationEmailAsync(TenantId, "ops@contoso.com");
+
+        repo.Verify(r => r.SaveNotificationEmailAsync(TenantId, "ops@contoso.com"), Times.Once);
+    }
+
+    // ------------------------------------------------------------------
+    // Conditional write (TableConfigRepository) — the actual invariant
+    // ------------------------------------------------------------------
+
+    private sealed class RepoHarness
+    {
+        public TableConfigRepository Sut { get; }
+        public Mock<TableClient> Table { get; } = new();
+        public List<(TableEntity Entity, ETag IfMatch, TableUpdateMode Mode)> Updates { get; } = new();
+
+        public RepoHarness(TableEntity? stored, RequestFailedException? throwOnUpdate = null)
+        {
+            if (stored == null)
+            {
+                Table.Setup(c => c.GetEntityAsync<TableEntity>(
+                        It.IsAny<string>(), It.IsAny<string>(),
+                        It.IsAny<IEnumerable<string>>(), It.IsAny<CancellationToken>()))
+                    .ThrowsAsync(new RequestFailedException(404, "Not Found", "ResourceNotFound", null));
+            }
+            else
+            {
+                Table.Setup(c => c.GetEntityAsync<TableEntity>(
+                        It.IsAny<string>(), It.IsAny<string>(),
+                        It.IsAny<IEnumerable<string>>(), It.IsAny<CancellationToken>()))
+                    .ReturnsAsync(Response.FromValue(stored, Mock.Of<Response>()));
+            }
+
+            Table.Setup(c => c.UpdateEntityAsync(
+                    It.IsAny<TableEntity>(), It.IsAny<ETag>(),
+                    It.IsAny<TableUpdateMode>(), It.IsAny<CancellationToken>()))
+                .Returns<TableEntity, ETag, TableUpdateMode, CancellationToken>((e, tag, mode, _) =>
+                {
+                    if (throwOnUpdate != null)
+                        throw throwOnUpdate;
+                    Updates.Add((e, tag, mode));
+                    return Task.FromResult(Mock.Of<Response>());
+                });
+
+            var serviceClient = new Mock<TableServiceClient>();
+            serviceClient.Setup(c => c.GetTableClient(It.IsAny<string>())).Returns(Table.Object);
+
+            Sut = new TableConfigRepository(
+                new TableStorageService(serviceClient.Object, NullLogger<TableStorageService>.Instance),
+                NullLogger<TableConfigRepository>.Instance);
+        }
+    }
+
+    private static TableEntity StoredConfig(string? contactEmail, string etag = "W/\"stored\"")
+    {
+        var entity = new TableEntity(TenantId, "config") { ["DomainName"] = "contoso.com" };
+        if (contactEmail != null)
+            entity["ContactEmail"] = contactEmail;
+        entity.ETag = new ETag(etag);
+        return entity;
+    }
+
+    [Fact]
+    public async Task Repo_seeds_only_the_contact_field_and_only_conditionally()
+    {
+        var harness = new RepoHarness(StoredConfig(contactEmail: null));
+
+        Assert.True(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "  ops@contoso.com "));
+
+        var (entity, ifMatch, mode) = Assert.Single(harness.Updates);
+        // Merge, not Replace: every other stored property must survive untouched.
+        Assert.Equal(TableUpdateMode.Merge, mode);
+        Assert.Equal(new ETag("W/\"stored\""), ifMatch);
+        Assert.Equal("ops@contoso.com", entity["ContactEmail"]);
+        Assert.False(entity.ContainsKey("DomainName"));
+    }
+
+    [Fact]
+    public async Task Repo_never_overwrites_an_address_the_tenant_already_owns()
+    {
+        var harness = new RepoHarness(StoredConfig("owner@contoso.com"));
+
+        Assert.False(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"));
+
+        Assert.Empty(harness.Updates);
+    }
+
+    [Fact]
+    public async Task Repo_treats_a_whitespace_only_stored_value_as_absent()
+    {
+        var harness = new RepoHarness(StoredConfig("   "));
+
+        Assert.True(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"));
+
+        Assert.Single(harness.Updates);
+    }
+
+    [Fact]
+    public async Task Repo_loses_the_race_rather_than_clobbering_a_concurrent_write()
+    {
+        // Someone wrote the row between our read and our write. The ETag precondition turns
+        // that into a 412; the seed abandons rather than overwriting the winner.
+        var harness = new RepoHarness(
+            StoredConfig(contactEmail: null),
+            throwOnUpdate: new RequestFailedException(412, "Precondition Failed", "UpdateConditionNotSatisfied", null));
+
+        Assert.False(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"));
+    }
+
+    [Fact]
+    public async Task Repo_with_no_configuration_row_reports_no_seed()
+    {
+        var harness = new RepoHarness(stored: null);
+
+        Assert.False(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"));
+
+        Assert.Empty(harness.Updates);
+    }
+
+    [Fact]
+    public async Task Repo_is_fail_soft_on_a_storage_fault()
+    {
+        // Callers are side effects of an already-committed write — a lost seed is recoverable
+        // on the next maintenance run, a thrown exception is not.
+        var harness = new RepoHarness(
+            StoredConfig(contactEmail: null),
+            throwOnUpdate: new RequestFailedException(503, "Server Busy"));
+
+        Assert.False(await harness.Sut.TrySeedTenantContactEmailAsync(TenantId, "ops@contoso.com"));
+    }
+
+    [Theory]
+    [InlineData(null, "ops@contoso.com")]
+    [InlineData("", "ops@contoso.com")]
+    [InlineData(TenantId, null)]
+    [InlineData(TenantId, "   ")]
+    public async Task Repo_rejects_incomplete_input_without_touching_storage(string? tenantId, string? email)
+    {
+        var harness = new RepoHarness(StoredConfig(contactEmail: null));
+
+        Assert.False(await harness.Sut.TrySeedTenantContactEmailAsync(tenantId!, email!));
+
+        Assert.Empty(harness.Updates);
+        harness.Table.Verify(c => c.GetEntityAsync<TableEntity>(
+            It.IsAny<string>(), It.IsAny<string>(),
+            It.IsAny<IEnumerable<string>>(), It.IsAny<CancellationToken>()), Times.Never);
+    }
 }

src/Backend/AutopilotMonitor.Functions.Tests/TenantOffboardFunctionTests.cs

@@ -87,7 +87,11 @@ public sealed class TenantOffboardFunctionTests
         return new PreviewWhitelistService(
             configRepoMock.Object,
             cache,
-            NullLogger<PreviewWhitelistService>.Instance);
+            NullLogger<PreviewWhitelistService>.Instance,
+            new TenantConfigurationService(
+                configRepoMock.Object,
+                NullLogger<TenantConfigurationService>.Instance,
+                cache));
     }
 
     /// <summary>

src/Backend/AutopilotMonitor.Functions/DataAccess/TableStorage/TableConfigRepository.cs

@@ -71,6 +71,54 @@ namespace AutopilotMonitor.Functions.DataAccess.TableStorage
             }
         }
 
+        /// <summary>
+        /// Conditional, single-property seed of ContactEmail (see <see cref="IConfigRepository"/>).
+        /// Merge mode leaves every other stored property untouched, and the ETag precondition turns
+        /// a concurrent write into a 412 rather than a silent overwrite. Both matter: the caller is
+        /// a background/side-effect path that must never beat a tenant admin's own edit.
+        /// </summary>
+        public async Task<bool> TrySeedTenantContactEmailAsync(string tenantId, string email)
+        {
+            if (string.IsNullOrWhiteSpace(tenantId) || string.IsNullOrWhiteSpace(email))
+                return false;
+
+            try
+            {
+                var existing = await _tenantConfigTableClient.GetEntityAsync<TableEntity>(tenantId, "config");
+
+                // The tenant already owns an address — the seed is one-way and never re-syncs.
+                if (!string.IsNullOrWhiteSpace(existing.Value.GetString("ContactEmail")))
+                    return false;
+
+                var patch = new TableEntity(tenantId, "config")
+                {
+                    ["ContactEmail"] = email.Trim()
+                };
+
+                await _tenantConfigTableClient.UpdateEntityAsync(
+                    patch, existing.Value.ETag, TableUpdateMode.Merge);
+                return true;
+            }
+            catch (RequestFailedException ex) when (ex.Status == 404)
+            {
+                // No configuration row yet — nothing to seed onto.
+                return false;
+            }
+            catch (RequestFailedException ex) when (ex.Status == 412)
+            {
+                // Someone wrote the row between the read and the write. They win by construction:
+                // a lost seed is recoverable (the backfill retries), a lost admin edit is not.
+                _logger.LogInformation(
+                    "Contact address seed for {TenantId} skipped — the configuration changed concurrently", tenantId);
+                return false;
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Error seeding contact address for tenant {TenantId}", tenantId);
+                return false;
+            }
+        }
+
         public async Task<List<TenantConfiguration>> GetAllTenantConfigurationsAsync()
         {
             try

src/Backend/AutopilotMonitor.Functions/Functions/Config/UpdateTenantConfigurationFunction.cs

@@ -76,6 +76,19 @@ namespace AutopilotMonitor.Functions.Functions.Config
                     return badRequest;
                 }
 
+                // The contact address is where enforcement actions and service notices are sent,
+                // so it must be an address rather than whatever string a direct API caller posts.
+                var contactEmailError = ValidateContactEmail(config.ContactEmail);
+                if (contactEmailError != null)
+                {
+                    var badRequest = req.CreateResponse(HttpStatusCode.BadRequest);
+                    await badRequest.WriteAsJsonAsync(new { success = false, message = $"Invalid contact email: {contactEmailError}" });
+                    return badRequest;
+                }
+                // Normalize so the stored value never carries surrounding whitespace, and an
+                // all-whitespace submission clears the field instead of masquerading as a value.
+                config.ContactEmail = string.IsNullOrWhiteSpace(config.ContactEmail) ? null : config.ContactEmail.Trim();
+
                 // Load the stored config up-front so we can (1) restore any redacted secret placeholders
                 // before validation/save and (2) protect GA-only fields below.
                 var existingConfig = await _configService.GetConfigurationAsync(requestCtx.TargetTenantId);
@@ -251,6 +264,52 @@ namespace AutopilotMonitor.Functions.Functions.Config
         private const int MaxCustomHeaderCount = 25;
         private const int MaxNotificationChannelsJsonLength = 65536;
 
+        // RFC 5321 caps a forward path at 254 characters.
+        internal const int MaxContactEmailLength = 254;
+
+        /// <summary>
+        /// Validates the tenant contact address. Returns an error message, or null when valid/empty.
+        /// Empty is legitimate — it means we have no way to reach the tenant.
+        /// <para>
+        /// Deliberately not an RFC 5322 parser: the job is to reject values that are not addresses
+        /// at all. Specifically it rejects recipient lists (a comma would silently widen who receives
+        /// service notices), display-name forms, and control characters (which would let a caller
+        /// forge mail headers once this address is actually mailed).
+        /// </para>
+        /// </summary>
+        internal static string? ValidateContactEmail(string? email)
+        {
+            if (string.IsNullOrWhiteSpace(email))
+                return null;
+
+            var trimmed = email.Trim();
+
+            if (trimmed.Length > MaxContactEmailLength)
+                return $"must be at most {MaxContactEmailLength} characters.";
+
+            foreach (var ch in trimmed)
+            {
+                if (char.IsControl(ch))
+                    return "must not contain control characters.";
+                if (char.IsWhiteSpace(ch) || ch == ',' || ch == ';' || ch == '<' || ch == '>')
+                    return "must be a single address, without spaces, separators or angle brackets.";
+            }
+
+            var at = trimmed.IndexOf('@');
+            if (at <= 0 || at != trimmed.LastIndexOf('@') || at == trimmed.Length - 1)
+                return "must contain a single \"@\" with text on both sides.";
+
+            // A bare host with no dot is unreachable from our sender, so it is a typo, not an address.
+            var domain = trimmed.Substring(at + 1);
+            if (!domain.Contains('.') || domain.StartsWith(".", StringComparison.Ordinal)
+                || domain.EndsWith(".", StringComparison.Ordinal))
+            {
+                return "the domain part must be a dotted host name.";
+            }
+
+            return null;
+        }
+
         /// <summary>
         /// Validates the notification-channel list JSON. Returns an error message, or null when
         /// valid/empty. Strict counterpart of the fail-soft <c>NotificationChannel.ParseList</c>:

src/Backend/AutopilotMonitor.Functions/Services/MaintenanceService.Aggregation.cs

@@ -527,6 +527,13 @@ namespace AutopilotMonitor.Functions.Services
         /// the seed happens when the notification address is saved, so this converges to a
         /// no-op and can be removed once every existing tenant has been covered.
         /// </para>
+        /// <para>
+        /// The enumerated snapshot is only a cheap pre-filter; the authoritative "is it still
+        /// empty" check happens under an ETag inside
+        /// <see cref="TenantConfigurationService.TrySeedContactEmailAsync"/>. A full-model save
+        /// here would write a snapshot that is minutes old by the time the loop reaches this
+        /// tenant, silently reverting whatever an admin changed in between.
+        /// </para>
         /// </summary>
         /// <returns>Number of tenants that received a contact address.</returns>
         private async Task<int> BackfillTenantContactEmailsAsync()
@@ -546,9 +553,8 @@ namespace AutopilotMonitor.Functions.Services
                     if (string.IsNullOrWhiteSpace(email))
                         continue;
 
-                    config.ContactEmail = email.Trim();
-                    await _tenantConfigService.SaveConfigurationAsync(config);
-                    filled++;
+                    if (await _tenantConfigService.TrySeedContactEmailAsync(config.TenantId, email))
+                        filled++;
                 }
 
                 if (filled > 0)

src/Backend/AutopilotMonitor.Functions/Services/PreviewWhitelistService.cs

@@ -16,16 +16,19 @@ public class PreviewWhitelistService
     private readonly IConfigRepository _configRepo;
     private readonly IMemoryCache _cache;
     private readonly ILogger<PreviewWhitelistService> _logger;
+    private readonly TenantConfigurationService _tenantConfigService;
     private static readonly TimeSpan CacheDuration = TimeSpan.FromMinutes(5);
 
     public PreviewWhitelistService(
         IConfigRepository configRepo,
         IMemoryCache cache,
-        ILogger<PreviewWhitelistService> logger)
+        ILogger<PreviewWhitelistService> logger,
+        TenantConfigurationService tenantConfigService)
     {
         _configRepo = configRepo;
         _cache = cache;
         _logger = logger;
+        _tenantConfigService = tenantConfigService;
     }
 
     /// <summary>
@@ -104,42 +107,15 @@ public class PreviewWhitelistService
     /// <summary>
     /// Saves (or clears) the notification email for a tenant, and seeds the tenant's
     /// contact address from it the first time one is given.
+    /// <para>
+    /// The seed is a side effect of a write that has already been persisted, so it must never
+    /// fail this call — <see cref="TenantConfigurationService.TrySeedContactEmailAsync"/> is
+    /// fail-soft and owns the "never overwrite what the tenant owns" invariant.
+    /// </para>
     /// </summary>
     public async Task SaveNotificationEmailAsync(string tenantId, string? email)
     {
         await _configRepo.SaveNotificationEmailAsync(tenantId, email);
-        await SeedContactEmailAsync(tenantId, email);
-    }
-
-    /// <summary>
-    /// One-way seed: copies the preview notification address into
-    /// <see cref="TenantConfiguration.ContactEmail"/> only while that field is still empty.
-    /// Once the tenant owns a contact address — seeded or edited in the portal — this never
-    /// touches it again, so a later change here cannot silently overwrite the tenant's choice.
-    /// Best-effort: the notification email has already been persisted by the caller, and a
-    /// failure to seed must not fail that write.
-    /// </summary>
-    private async Task SeedContactEmailAsync(string tenantId, string? email)
-    {
-        if (string.IsNullOrWhiteSpace(email))
-            return;
-
-        try
-        {
-            var config = await _configRepo.GetTenantConfigurationAsync(tenantId);
-            if (config == null || !string.IsNullOrWhiteSpace(config.ContactEmail))
-                return;
-
-            config.ContactEmail = email.Trim();
-            await _configRepo.SaveTenantConfigurationAsync(config);
-
-            _logger.LogInformation(
-                "Seeded tenant contact address for {TenantId} from the preview notification email", tenantId);
-        }
-        catch (Exception ex)
-        {
-            _logger.LogWarning(ex,
-                "Could not seed tenant contact address for {TenantId} — the notification email was still saved", tenantId);
-        }
+        await _tenantConfigService.TrySeedContactEmailAsync(tenantId, email);
     }
 }

src/Backend/AutopilotMonitor.Functions/Services/TenantConfigurationService.cs

@@ -125,6 +125,48 @@ namespace AutopilotMonitor.Functions.Services
             }
         }
 
+        /// <summary>
+        /// One-way seed of the tenant contact address: writes <paramref name="email"/> only while
+        /// the tenant has none, and only if nothing else wrote the configuration in the meantime.
+        /// Returns true when the seed landed.
+        /// <para>
+        /// Single owner for both seed paths (preview notification-email save and the maintenance
+        /// backfill), because both need the same three things: the conditional single-property
+        /// write, the "never overwrite what the tenant owns" invariant, and the cache invalidation
+        /// below — the repository writes behind this cache, so without it a freshly seeded address
+        /// stays invisible for the 5-minute TTL.
+        /// </para>
+        /// <para>
+        /// Fail-soft by design, unlike <see cref="SaveConfigurationAsync"/>: every caller is a side
+        /// effect of an operation that has already succeeded, and a lost seed is recoverable on the
+        /// next maintenance run. Enforced here rather than left to the repository, so the guarantee
+        /// holds for any implementation.
+        /// </para>
+        /// </summary>
+        public virtual async Task<bool> TrySeedContactEmailAsync(string tenantId, string? email)
+        {
+            if (string.IsNullOrWhiteSpace(tenantId) || string.IsNullOrWhiteSpace(email))
+                return false;
+
+            try
+            {
+                var seeded = await _configRepo.TrySeedTenantContactEmailAsync(tenantId, email!.Trim());
+                if (seeded)
+                {
+                    _cache.Remove($"tenant-config:{tenantId}");
+                    _logger.LogInformation("Seeded contact address for tenant {TenantId}", tenantId);
+                }
+
+                return seeded;
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex,
+                    "Could not seed contact address for tenant {TenantId} — the triggering write still stands", tenantId);
+                return false;
+            }
+        }
+
         /// <summary>
         /// Invalidates cache for a tenant (forces reload on next request)
         /// </summary>

src/Shared/AutopilotMonitor.Shared/DataAccess/IConfigRepository.cs

@@ -24,6 +24,21 @@ namespace AutopilotMonitor.Shared.DataAccess
         /// </summary>
         Task<RawPage<TenantConfiguration>> GetTenantConfigurationsPageAsync(int pageSize, string? continuation);
 
+        /// <summary>
+        /// Writes <paramref name="email"/> to the tenant's ContactEmail ONLY while that field is
+        /// still empty, and only if nothing else wrote the row in the meantime. Returns true when
+        /// the seed landed, false when the tenant already owns an address, has no config row, or
+        /// lost the race.
+        /// <para>
+        /// Exists because the seed cannot be expressed as a read-modify-write of the whole model:
+        /// <see cref="SaveTenantConfigurationAsync"/> replaces the entire row unconditionally, so a
+        /// concurrent portal save would be clobbered by the seeder's stale snapshot — of every
+        /// field, not just this one. The implementation must therefore write conditionally and
+        /// touch no other property.
+        /// </para>
+        /// </summary>
+        Task<bool> TrySeedTenantContactEmailAsync(string tenantId, string email);
+
         // --- Admin Configuration ---
         Task<AdminConfiguration?> GetAdminConfigurationAsync();
         Task<bool> SaveAdminConfigurationAsync(AdminConfiguration config);