There have been numerous reports of problems using Apple's Mail program with Comcast. I have Comcast, but use Eudora and have never had a problem.
These are a couple items from MacFixIt;
Our investigation into Comcast mail retrieval issues (caused in some cases by problematic messages on the server that can be deleted through the Web interface) also led us to a potential problem with Mail.app when encountering certain types of large messages.
So it appears that there may be some messages that cannot be properly interpreted by Mail.app when using a POP account. If you think you might be affected by this issue, try logging into the Web interface for e-mail access (most Internet service providers offer this service) and check for mail that you have not received in your regular client.
Other readers report that rebuilding the Mail.app database (go to Mailbox > Rebuild) after deleting mail messages from the server was required for new mail to appear properly.
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All Comcast users should make sure that if they are using Mail.app and receive large amounts of email or email with large attachments, to change the default preferences for deleting mail of off the server. The server can fill up and easily overrun your account limit. If you go over your limit you will be unable to receive mail or even send mail until your mailbox size is under the limits set by comcast...10MB per account. If you have relatives that send you pictures of their last vacation to French Lick, Indiana your mailbox space could disappear, as the saying goes, "with the quickness"