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Lots of ISPs does this. They require you to relay via their SMTP-server.

Ideally you would use port 587, which is the designated port for mail submission. See RFC 2476 [1].

This requires that your mail provider supports that port, of course, which is unfortunately not always the case.

Used to have Bellsouth DSL, and they did this. SMTP/TLS on a different port worked though. But obviously you couldn't run your own e-mail server.