[sliverstorm]

If you block 25 outbound, people's mail clients (Outlook, Thunderbird, etc) could not deliver mail to servers that don't run SSL or TLS (465, 587)

[werid]

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

[ckuehl]

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.

[1] http://www.ietf.org/rfc/rfc2476.txt

[daigoba66]

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.