For better helping you with your problem you will have to provide us a lot more details. What email client are you using? Are you getting any error message if so what does the exact error message? Any other more info you can provide would be helpful.
Also do post which settings you are using.... For POP 3 and for SMTP.
Have you cleared your browser cache? Which browser?
Need Email Help? Please post the following information in your post. Do you use XfinityConnect? The Full or Lite version? Do you use an email client? Which one? (Eg; Windows Live mail, Outlook, a smartphone etc.) Which browser/version do you use? And- have you cleared your browser cache? Which operating system? XP, Vista, Windows 7, Mac OS X Details of the problem you are having.
I also use Mac Mail. Sometimes it's just finicky and the handshaking didn't work. So I try again.
Sometimes the address is odd, so I usually check whether there's a missing .com or something.
Sometimes sending a message to myself nudges the mail server and things get going again.
If it doesn't work after a few minutes, you can check the mail settings and particularly the outgoing settings where you should set the port to 465 or 587 (choose one, not the default that may wander into 25). Click in front of SSL, set authentication to password and enter your username (without the @comcast.net) and your password.
If you also have problems with incoming mail, set that port to 995, use SSL and password authentication.
The password needs to be set in two places when you change it.