Below are the troubleshooting steps you can follow if external recipient not receiving emails from your Exchange server:

1. Verify that the message sent has not been blocked by your mail server
2. Check the SPF, DKIM, DMARC records of the domain sending the message
3. Verify DNS resolution on the sender and recipient mail server
4. Check the mail server log to check for delivery error
5. Utilize mail testing services such as mail-tester.com to assess the message
6. Validate the email addresses of the recipients