SMTP Port Blocking.

These days many ISP's block port 25 which is used for outgoing SMTP mail. This is an increasing trend in the ongoing way to reduce SPAM but can also stop legitimate users from being able to send their mail out through other SMTP servers.

The solution is to use an authenticated SMTP mail service that supports alternate SMTP port mail relaying - for instance using port 225 or 2525.

To solve this have a look at AuthSMTP.