No route was found for the recipient server.
I saw a very weird problem this past week on a system running Exchange Server 2003. The organization has 2 separate Exchange organizations/forests. They are in the middle of migration from OldDomain.com to NewDomain.com and are using the Quest Migration tools.
Many of the users had mailboxes in the new domain, but their new domain mailbox had an alternate recipient that forwarded all mail to a contact in olddomain.com.
When Quest first sync'ed the "old" mailboxes with the "new" mailboxes, it removed the alternate recipient and added to the new mailbox a targetAddress of UserDomain@source.olddomain.com.
The problem is that for ONE user out of hundreds, anytime that someone sent him a message, they got back HUNDREDS of NDR messages. The user had not yet been migrated to the new domain, so the targetAddress attribute was checked and it was correct. NDR message was:
No route was found for the recipient server. Please contact your system administrator.
The message was originating from the sender's mailbox server. After lots of examining logs and message tracking, we found that the message WAS actually getting to the intended recipient, but the NDRs just kept coming in.
Ultimately for this one user, in the new domain on the alternate recipient forward, the checkbox to send to both the mailbox AND the alternate recipient was checked. But, the alternate recipient address was now blank. We had to enable a forwarder temporarily, clear that checkbox, and then remove the forwarder.