Review all your settings to make sure they agree with
http://www.comcast.com/Customers/FAQ/FaqDetails.ashx?Id=2288
In particular, note that the outgoing server now uses port 587 rather than 25.
The receive error you included below is not very revealing as to the
cause of the problem. If you are sure you are using the correct username
and password, one possible cause for that error is account corruption.
One thing you might try as a possible quick fix is to delete that corrupted
account, restart Windows Mail, then recreate that mail account.
If that doesn't fix it, more drastic measures will have to be taken.
Which antivirus are you running?
As a minimum, email scanning in the antivirus must be turned off,
although that may not be sufficient to eliminate all bad effects.
In a worst case scenario, your antivirus may need to be uninstalled.
If turning off the email scanning function does not resolve your problem,
consider upgrading to Windows Live Mail, because it is less prone to
suffering bad effects from overly aggressive antivirus programs.
http://get.live.com/wlmail/overview
Email scanning in any antivirus must be disabled, for reasons
explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm