Go to Edit and Mail & Newsgroups Account Settings...
 
 
 

go to Outgoing Server (SMTP) in the left-hand column, at the bottom.  Your server name should he whatever comes after the @ sign in your e-mail address.  Example: joe@pcinw.com,. your Server Name would be pcinw.com  You must make sure that use name and password is checked and that the User Name is your entire e-mail address.  Click on OK and your done.