I have specific customers whose emails are not getting through to my Exchange 2003. They request a Delivery receipt and get it, but the email doesn't show up in logs or in the mailboxes. Any help would be greatly appreciated.

Hi I recommend you enable Messsage Tracking and track messages that sent to you , so you can see what happened to these messages. Osman Shener

I have used Message Tracking and the messages do not appear in Message Tracking.

But you can see other senders messages , arent you ? Couldyou check your SMTP Logs , you can search sender email address or sender server in your log files to see what happened. If you cant find something , could you describe your messaging architecture components and flow. (Like Internet Connection-Router-Firewall-MailControl-ExcXX-ExcXX)