Confirmation of receipt of an email from an automated response

Receiving an automated response is strong evidence that the original email was successfully sent and at least processed by the recipient’s mail system, but it is not an absolute guarantee that it ended up in (and stayed in) the person’s inbox.

In typical mail flow:

• For an automatic reply (for example, an Out of Office/OOF reply) to be generated, the message normally has to be accepted and processed by the recipient’s mailbox rules on the server.
• After that, several things can still happen that prevent the user from seeing it:
  • The message is detected as spam and moved to junk or quarantine.
  • A rule or policy deletes or moves the message after delivery.
  • The message is sent for moderation or otherwise delayed.
  • The destination mailbox later rejects or times out on the message in a way that still leaves some server-side processing traces.

Exchange Online documentation lists multiple reasons why a message might not be received even when the service has processed it, including spam filtering, quarantine, rules, malware filtering, moderation, and delivery failures or timeouts. It recommends running a message trace and checking the delivery status (for example, Failed or Pending) to determine exactly what happened to the message and whether it was filtered, redirected, or deleted after acceptance.

So:

• An automatic response makes it very likely that the message reached the recipient’s mail system and triggered server-side logic.
• It does not absolutely prove that the message is (or remains) visible in the recipient’s inbox.
• The exact likelihood depends on the organization’s spam filters, rules, and policies; only a message trace on the recipient’s system can definitively show the path and final status of the email.

References:

• Message Trace FAQ in Exchange Online
• Understanding and troubleshooting Out of Office (OOF) replies