I have turned off all filters and block lists both on Exchange and ISA server yet some mail is still never delivered to recipients. I can see in the SMTP logs that, say, someone@hotmail.com had a successful session with my server but the mail will never get delivered to the recipient. It just disappears. No NDR goes back to someone@hotmail.com either. There are a number of other domains that send mail in that doesn't get distributed by the Exchange server, not just one particular domain. I can sucessfully telnet to the server and send mail in as someone@hotmail.com and this mail is immediately delivered so the problem must have something to do with the IP of the sender and not the name of the domain that causes the message to disappear. Any ideas? I'm about ready to throw up my hands and call Microsoft.