Who's your host? My primary VPS host and my home connection both block the only port I'm supposed to use for mail.

(A curiosity to me, as someone trained on HTTPS, which doesn't really care what port you're coming from. I wish there was just mail-over-HTTPS, since port 443 can't be blocked by a reasonable host.)

A lot of VPS hosts will unblock the port for you on request. I think that is better than open by default as that attracts spammers and gives IPs a bad reputation for future users.