Comcast, as well as many other ISPs have started to block Port 25 to send email. They block this port in your modem, so it doesn't matter if you are using their mail servers or ours, you still won't be able to send through Port 25.
They should send you an email with instructions on how to change to port 587. Some Comcast subscribers have stated they didn't receive this email. You can also find the information on Comcast's help boards on their website (comcast.net).
We have configured our servers to allow mail to be sent through port 587, so if you modify your accounts according to their instructions, you should be able to send email again.
