Separate submission from delivery
After submission, recipient servers apply their own acceptance, filtering, and mailbox rules. A message can be delayed, rejected later, quarantined, or filed outside the inbox even when the initial send step reports success.
Confirm that the address is valid and that the message was addressed to the intended recipient. For a shared mailbox or alias, check the destination's routing and access settings with its administrator.
Troubleshoot without creating duplicates
Look for a bounce or delivery status associated with the message and use the provider's event details to identify whether the recipient server accepted or rejected it. If the evidence is unclear, send a controlled test to an address you can inspect.
Do not immediately resend to a large list: recipients may receive duplicates if the first message was delayed. Correct address, authentication, or policy issues first, then retry selectively and monitor the result.