I also have Comcast and my iPhone won't get ANY of my emails anymore. Worked fine ever since I got the Phone over a year ago. I just updated to Outlook 2007 so I am guessing that is where the issue is but all the settings are correct on the phone. I don't know what the hell is going on.
Be sure your Outgoing server is set correctly according to your service provider (when on wifi!).
While I don't use a cox or comcast email, I do use one hosted from my own website. Emails always receive fine, but when Im connected via wifi, and attempted to send an email, they would never go through, because cox specifically requires all outgoing email to go through their own outgoing server. Its so annoying.