I have one user that when anyone sends him an email an Undeliverable Email Rejection comes back from the Administrator even though the email is delivered to the user. I have tried using his default email address along with another alias associated with his account. Message below: Your message did not reach some or all of the intended recipients. Subject: Test Email Sent: 8/27/2009 2:01 PM The following recipient(s) cannot be reached: XXXXXX on 8/27/2009 2:01 PM The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.<XXXXXXX.xxxx.local #5.1.1>

Check for any rules to see if some forwarding is going on. ( Check his account as well).Any delegates?

Also, can you confirm that this happens for all mails or just for meeting request. For meeting requests, it would be because of delegates as Andy said, check the settings or a hidden delegate rule in mailbox through MFCMapi as explained in below similar thread... http://social.technet.microsoft.com/Forums/en-US/exchangesvrgeneral/thread/8ef3fa80-7692-4402-b788-39774a2e81e9Amit Tank | MVP Exchange Server | MCITP: EMA | MCSA: M | http://ExchangeShare.WordPress.com