Postfix "loops back to myself" only when trying to send to fallback relay

Solution 1:

According to a post I found in a forum somewhere, using any port other than port 25 will disable the mail loop detection that I was running into. However, I have been unable to find any authoritative source for either why the mail was considered looping or why changing the connection port made a difference. I will leave this question open for an answer that points to authoritative sources.

The solution that worked for me was to add TLS certificates and enable SMTP and TLS on port 587 on deferred-mail.example.com and change the configuration on mail-server.example.com to

smtp_fallback_relay = [deferred-mail.example.com]:587

I think this is more than is necessary; the post said any port other than 25 would do. I did it this way because I wanted to correctly set up TLS on port 587 anyway.