1) Goto Edit | Preferences | Identity
2) In "Email Address", enter [username]@[yourdomain].com
3) In "Reply to...", enter whatever@[yourdomain].com
4) Goto Mail&Group
5) In "Mail server user name", enter [username]
6) Outgoing Mail(SMTP)* server: mail.[yourdomain].com
7) Incoming Mail(POP3 or IMAP)server: mail.[yourdomain].com
8) Click "OK"

*Using SMTP Server

Per dialup session, you would need to be authenticated to use the SMTP(Outgoing Mail) server. This is to prevent spammers from gaining access to your SMTP server. Here is how you can be authenticated:

1) Get connected to the Internet
2) Start your email software.
3) Make sure there is no outgoing mail in your outbox.
4) Log into the POP server to check your email
5) Now you are authenticated and you should be able to send and receive email w/o any problems. As long as you don't quit the program and stay connected to the net, you wouldn't need to be authenticated anymore.

If your SMTP access is denied after trying the steps above, you would need to contact your ISP. It is likely that they are not allowing relay to outside SMTP server. More and more ISP's are implementing this feature to prevent spamming.

There is a very easy workaround to this. All you have to do is use your ISP's SMTP server. This does not affect how your email address appears on your outgoing emails. When recipients of your message hit the reply button, they will see your email address at your domain name.