Maybe its related to this

http://www.newswithviews.com/NWVexclusive/exclusive111.htm

At first the supervisor denied Comcast blocks e-mail. It was during her discussion with the Comcast supervisor that he finally admitted they do block e-mail they consider spam and unwelcome by the recipients.
According to Deanna Spingola, the Comcast supervisor said that his company makes the decision whether or not to treat e-mail as spam.

EDIT: As a test, try using an alternate email provider (there are several free ones) to send an email you know was previously not received. Have someone send a similar email to your alternate address. If these are received I would suspect Comcast. Otherwise check your spam filter settings to see if the option to automatically delete spam is enabled since this option would not give you the opportunity to read the email before deletion.