Open MAC OS X Mail. Click Mail -> Preferences. Click Accounts to bring up the accounts preferences panel. Click the + (lower/left highlighted in green) to add an account. Enter the information (e-mail address, incoming server, and password.)
(User Name= your email address + yourwebsite.com) (Incoming and Outgoing server=mail.yourwebsite.com) (Email firstname.lastname@example.org)
NOTE: All e-mail addresses and logon information are completely case sensitive
To enter the Outgoing mail server, click the arrows near Outgoing mail Server (SMTP), highlighted in green and click Add Server. The outgoing server is: mail.yourwebsite.com and it has to be authenticated with password.