Relay access denied when using SMTP to external recipients POSTFIX FREEBSD

Isn't that you haven't authenticated properly (via your mail client) to send outbound email or otherwise haven't setup mynetworks such that it will allow all systems within your network to send without authentication?

https://serverfault.com/questions/42519/how-to-correct-postfix-relay-access-denied

I had a similar error message when I set up sendmail as in this thread. For me the problem was that I was authenticating over port 465, which is SMTP with TLS, but the protocol requires TLS to run after authentication, for example over port 25, which is SMTP without TLS.

The solution was to change the port in ~/.authinfo to

machine server.domain.tld login [email protected] port 25 password XYZ

Update: I'm no SMTP expert. My answer is based on my experience and on this thread, where a comment mentioned this rebuttal about my use of port 25 for authentication:

That's not the way SMTP AUTH works: you authenticate and send using the same port, in this case 587.