E-Mail Problems

I am running exchange server 2003 its seem that sometimes users can not send email to certain addresses including AOL and Comcast and some others I have not the slightest clue why this is happening any help would be great
Thanks alot

this is the error messeage

Your message did not reach some or all of the intended recipients.

The folowing recipients(s) could not be reached

Tanya on 12/13/2006
Could not deliver the message in the time limit specifided. Please retry or contact your administrator