Make sure your CSS is inline rather than in HEAD or BODY tags.
Browser-based email applications (Yahoo, AOL, Hotmail) strip CSS out of HEAD and BODY tags by default, so unlike web browsers, many email clients won’t properly render CSS inside HEAD and BODY tags.
[note title=”Note”]Since BODY tags are stripped, in order to change background color of an email, you must put a 100 percent wide TABLE wrapper around the mail.[/note]
For extensive information about what works and doesn’t work for various email clients, click here.