There are now a large number of ISP's that are starting to block Port 25. This is done to prevent spamming, which is great!!! However, those of you who may have your own domains can run into a problem --- you can receive mail but you cannot send mail through your mail server.
I know some of you may be using sendmail. This is not guaranteed to work for all recipients since ISP's are now perfroming reverse lookups on the senders. They may block it or send a response sayung that at some future date they will no longer accept your e-mails.
You need to use your ISP's SMTP server! Now all you need to do is setup so that you are pointing to your ISP's SMTP server, but still use your own doamin name userid. (i.e. me@mydomain and myispsmtp@myiispdoamin)
Here is a link to determine if your ISP is blocking port 25 http://www.kwsonline.com/rcenter/ispconfig.htm