Click on the link below and select your computer's email software from the drop-down menu.  Then click on the "IMAP" or "POP" options to see the instructions and mail server settings.  If your website was launched before September 2009, use the instructions at the bottom of this article rather than clicking on the link below.

https://emailhelp.rackspace.com/#/

Make sure you have the full email entered as the username. Both the incoming and outgoing email servers should be listed as mail."your domain name" (e.g. mail.website.com). Also make sure that the setting for the outgoing server is set to: "outgoing server requires authentication". Then change the outgoing port number to 587. The incoming port should be set to 110.  Do not use SSL.