To help resolve issues with email delivery to external recipients in Exchange:

1. Verify that the domain MX record is set up and propagated properly for the recipient domain.


2. Verify the sending domain name in the message’s Return-Path header.


3. Ensure DNS resolution from the sending server to the recipient server.


4. Confirm that the recipient’s mailbox is not full.


5. Check for external content that may be blocked by the receiving server.


6. Check whether the size of the message exceeds allowed size limits.


7. Confirm the recipient mailbox exists.