I had to work through this problem recently. Many of our users wanted to access and send email from home. All could access their imap accounts but almost none to send emails... due to the fact that almost all isps now block port 25 to anywhere but their own server. To work around this I set up another smtp server to run on another port and configured the users to send email to that port... it all works great now. If you need help figuring out how to add another smtp server let me know.
FWIW, I always encourage my users to USE their ISP for outgoing SMTP. The only time I don't is when their ISP prevents them from using a different domain in their Return-Path. Limiting outgoing SMTP to the user's ISP is one of the better ways of tracking and preventing mail abuse since the ISP can track what account was actually using the offending IP address.
Regards,
Bill Shupp
