MAC-Apple Mail
Open the Apple Mail application. Click on the "Mail" drop down menu, and select the "Preferences..." option.
Select the "Accounts" option from the top bar. NOTE: If the top bar does not show as seen in the image, click on the grey oval button in the top right hand corner of the window, this will cause the top options to display. Under the "Outgoing Mail Server (SMTP):" option, click on the "Server Settings..." button. In the new pop-up enter your mail server, which should be mail.mydomain.com (replace "mydomain.com" with your domain name). Under "Authentication:" select Password. In the "User Name:" field enter the user name formatted in This email address is being protected from spambots. You need JavaScript enabled to view it. (replace account with the name or word that appears in front of the '@' symbol, and replace mydomain.com with your domain name). Enter your password in the password box.
Click "OK", then close the Account window by clicking on the red circle.