Go to the File | Info menu item, then click on Account Settings.
In the next dialog box (window), select the Email Account you wish to change, then click on Change ...
In the next dialog box (window), click on More Settings.
In the next dialog box (window), click on the Outgoing Server tab, then make sure that you click the checkbox entitled "My outgoing server (SMTP) requires authentication".
Click OK, then click Next, then click Finish in the Accounts Settings window, and complete the process by closing any remaining open windows.