Supported provider defaults could pass validation without enabling SMTP
authentication, and preparing concurrent sends could overwrite another
delivery's sender and recipients. Resolve credentials when the SMTP manager
is created and keep each delivery's configuration independent while sharing
the existing rate-limit state.
Verify real SMTP acceptance, invalid-credential dead letters, persisted
queue outcomes after restart, concurrent envelopes and shared rate limits.
rcourtman
bec3a10019