Mac OS X Lion Mail not sending
These problems usually come down to one of two things:


You have incorrectly configured the log-in settings for your outgoing mail server run by the company that provides your email service. Port numbers, authentication method, user name, password, and other settings.
The problem is on the end of the company that provides your email service. Their servers are misbehaving.


In either case, call the tech support line of the company that provides your email service and have them troubleshoot it with you.