problem:

 Get bounce messages like below:

 <user@domain.com>: host mail.domain.com[0.0.0.0]
    said: 550 5.7.1 Unable to relay for user@domain.com (in
    reply to RCPT TO command)

Appears from an Exchange server that was otherwise working correctly (Delivery connector was setup correctly and permissions were set correctly previously).

 Other errors appear in the Event Viewer:

"Receive connector 0.0.0.0:25 requires Transport Layer Security (TLS) before the MailFrom command can be run, but the server can't achieve it. Check the authentication settings of this connector."

If you telnet to the server port 25, you will notice the absence of "250-STARTTLS"

solution:

Check that the Windows SMTP Service is not running.  Stop the SMTP service and make sure when you telnet you get the STARTTLS feature announced now.  If this works, disable the service to prevent future conflicts.  You also may need to restart the Exchange Transport service to recover from the TLS errors in the Event Log.