We're experiencing an issue that doesn't seem to happen very often, so it's been difficult to troubleshoot. We've had an issue with a user sending out updates to calendar invites and it never making it resulting in an NDR.
When running message tracking, this is what we get
Date Time SMTP Store Driver: Message Submitted from Store
Date Time SMTP: Message Submitted to Advanced Queuing
Date Time SMTP: Started Message Submission to Advanced Queue
Date Time SMTP: Message Submitted to Categorizer
Date Time SMTP: Message Catergorized and Queued for Routing
Date Time SMTP: Message Routed and Queued for Remote Delivery
Two Days Later SMTP: Non-Delivery Report (NDR) Generated
So we're missing the line for SMTP: Started Outbound Transfer of Message.
The following line would normally show transferred message to server.domain.com through SMTP. We are using an encryption gateway setup as a smart host. All other email is flowing through properly. This just randomly seems to be an issue. This has also only happened to one person that I know of. Previously this was with sending to a gmail account. Now this is a private domain that this is happening. All other mail and original calendar invites are flowing properly. This is just calendar updates. This was also the case for the gmail issue.
We are running Exchange 2003 fully patched. Email comes into the organization through a spam firewall, and out from the exchange server to an encryption gateway and to the real world.
Please let me know if you need any further information.