This reminds me of something that happened with my wife a while ago. You may have already checked this, but have you looked at the address on the sent emails on her blackberry? That is, have you looked at the emails that have shown up in your comcast account and compared the "To" address to the sent email on her BB? I'm not sure how this would work if you've deleted that address from her contacts, but electrons do crazy things on occasion.
My wife was absolutely sure that she had been sending email to a certain address, but, in truth, she was just assuming that she did.