I get many email messages with the subject "Returned mail: User unknown"

The recipient's email address might not be valid. Maybe, the address is no longer in use. In this case, it cannot deliver message to the intended recipient and you get the Returned Mail messages.

Someone might have "spoofed" your email, making it look like you were the original sender of the message. This technique is used by spammers where they give your email address as "From" address, making it look like you are sending those messages. Use Report As Spam feature in your Comcast Webmail or send these messages to missed-spam@comcast.net.

Another reason could be, virus infected your system. Make sure to update your anti-virus and firewall software and run a full system scan.

If you are sure that your system does not have viruses, then virus might have infected your friend's machine and these people have your email address in their address box. In this case, the virus uses their address book to send messages. The virus usually uses a wrong address keeping your address as "From:" or "Reply-to:".