Some ISP's block the SMTP Relay causing outgoing messages to bounce. If this happens to you, follow the instructions below by changing the outgoing SMTP port settings.

Open "Apple Mail".
From the "Mail" menu, choose "Preferences".

From that window's toolbar, click "Accounts".
Highlight the email account on the left panel called "Your Email Account (in this demostration)".
Click on "Account Information" tab.
Click the "Server Settings" button.

Make sure "smtp.yourdomainname.com" in the "Outgoing Mail Server" field.
In the "Server port", type "26" in the field box.
Select "Password" under "Authentication".
Type in your email address in the "User Name" field.
Type in your password in the "Password" field.
Click "OK".

Close the Accounts window to save your settings.