Temporary Failure from mailer-daemon@comcast.net

At various times I've sent an email from my comcast account to a specific friend (also on comcast). The email goes through tot he friend, but I then receive Temporary Failure emails saying that "delivery to the following recipients was aborted"    The strange thing is that the recipient identified is a gmail account that I've never heard of (and neither has my friend).  This has happend before with the same friend, but with no other emails sent to other friends.  And it doesn't always happen with my friend, but when it does, the same unidentified and rejected email address is always the same.    The Temporary failure email sent back to me by the daemon has the mail attacahments showing my email and it looks all good.   Does anyone have any idea of what's going on here?