Prepare an acceptance test
Define the exact event, sender, recipients, attachment, expected timing, and success evidence before changing configuration. Record rollback values securely without copying secrets into tickets.
Test each stage
Verify DNS and TCP 587 from the client network, STARTTLS negotiation, authentication, matching envelope sender, SMTP DATA acceptance, the connection counter, Sent Items, Microsoft message trace where available, and final recipient delivery.
Exercise the real workflow
A plugin or device test may use a different event path. Trigger the real password reset, order state, scan, backup warning, infrastructure alert, or scheduled job.
Close the migration
Monitor expected and unexpected volume, update ownership and recovery documentation, remove the old settings, and revoke credentials that are no longer required.