Had a similar problem with my email being blocked and after several calls to Comcast,I was directed to their Security Dept. Was informed that they had somehow figured that because I was sending OE email with attachments to mutiple addresses using BCC's that somehow I was sending viruses. Despite my assurance that I did daily updates and scanned system for viruses weekly and all incoming and outgoing emails were being checked for viruses with the "free" McAfee Antivirus software provided by Comcast, they decided to block my outgoind emails.
I was then advised that there was a one click fix for the problem and it follows.
Third-Party Mail Client Users (Outlook, Outlook Express, etc.)
If you use Outlook Express, the steps we have taken to protect the Comcast network will not allow you to send email until you apply a simple one click fix available at http://www.comcastsupport.com/alternateport. While this will restore your ability to send mail it is still important to remove any possible viruses from your computer. This can be done by using the FREE McAfee Antivirus and Firewall software available from Comcast on the Comcast Security Channel .
This fixed the problem and I have since removed their free anti-virus softwre and installed AVG.
You may wish to consider the "one click fix".