Any time lately I try to send an email to various addresses at comcast.net, they are boundced back with "permanent error" and "Comcast block for spam" in the error message. The emails were perfectly harmless. What's the problem here?
This sounds like its a problem at the comcast end, its possible they have blocked your email/IP and labelled it as spam.
Are you able to send to other domains without any problems? If so I would recommend contacting comcast or getting your friend to so they can find out why and if they have blocked your address.
Once we get information from them we can then troubleshoot further from our end.