There are a number of possible causes for this. I’ll cover the ones I’m familiar with. Hopefully someone else in this group can pick up the slack.
This message generally means that the sender’s IP address has been blocked because they are spamming others.
Before I would assume that though, I’d ask if your friend is able to send email through his/her Comcast account to anyone.
The ability to send email through any of the web-based systems does not mean anything in this situation.
Some people get blocked because they have viruses or other pests on their systems which are using their system to send spam without the owner’s knowledge. If he/she has a Comcast account, then he/she should contact them for additional help.
Additionally, your friend should make sure that his/her anti-virus software is up to date, and that the full system has been scanned and cleaned. Likewise the system should also be cleared of spyware. Ad-Aware, Pest Patrol, Spy Sweeper, Microsoft Anti-Spyware, Spybot Search & Destroy (just to name a few) are all legitimate and useful tools. Some of them are free to boot.
It’s also possible (knowing Comcast) that your friend has picked up an IP address that was formerly used to spam with by someone else, and the block is still in place.
Another possibility is that the mail server address being used for outgoing mail either does not belong to Comcast, or is mis-configured.
I hope that helps. Write back with more details based on what I and others suggest and we may be able to help further.