I recently started having problems with outgoing email, and it turned out that my ISP has blocked port 25. Fortunately, Dreamhost* (as well as most other large providers, I’m sure) support RFC 2476, meaning you can use port 587 instead of 25 for outgoing email.
So if you’re experiencing problems sending email, try changing the SMTP port in your mailer from 25 to 587.
* that link contains a referral id, which earns me some cash if you sign up with Dreamhost after following the link.
Several ISPs have done this of late. It’s in an effort to prevent SPAM: http://www.online.no/kundeservice/20071012-telenor_spam.html
What kind of crazy ISPs does that? Isn\’t it then better if they just cut the whole internet, that will stop spam