Patricia- That would be the only possible explanation. Some spam filters are set to "trick" a sender into thinking the address is not valid when it really is if they suspect the message is spam. This is done by bouncing the presumed spam back with the User Unknown reason. If it only happened a few times randomly I'd say that perhaps Comcast was just having server problems -- but not if it CONSISTENTLY happens with forwarded mail and replies but not with new messages. That would indicate something in the filters. Joan In a message dated 10/4/2008 11:39:03 P.M. Eastern Daylight Time, [email protected] writes: In a message dated 10/4/2008 7:59:10 P.M. Pacific Daylight Time, [email protected] writes: This bounce would indicate that the address you sent the message to is NOT a valid Comcast address: [email protected]_ (mailto:[email protected]) .. BUT--if you thanks, Joan. Her address is correct, and I get mail to her just fine when it is not a reply or forward. Her son knows computers pretty well, so I will tell her to have him check the filters to see if that is the problem. It is really frustrating! Thanks for the hint! Patricia **************New MapQuest Local shows what's happening at your destination. Dining, Movies, Events, News & more. Try it out! (http://local.mapquest.com/?ncid=emlcntnew00000001)