Well, the first thing I would do is to switch to K-9 Email app:
Downloads - k9mail - Project Hosting on Google Code. The stock email apps aren't very good. But I don't think that is the root of your problem.
In most all cases the problem is with the port you are selecting and the security settings. A lot of isp's block port 25 and that is the default port on most of the email apps. So redo your settings in the manual mode and put in try port 465 or 587 and play around with different security settings. On Cox Cable we found, through trial and error because cable companies will quite often tell you that they don't provide support for cell phone usage, was that if we selected TLS (when available) the emails would go out. Also do a google search for "smtp port Comcast" and you'll get some other possibilities perhaps.