Message trace is one of the first places to look when you need to know whether Exchange Online received, rejected, deferred or delivered a message.
Start narrow
Use the sender, recipient and a useful time window. If you have message identifiers from headers or another system, keep them available for correlation.
Read events, not just final status
A delivered status does not necessarily mean the message landed in the Inbox. Trace details can show processing events and policy actions. Microsoft notes that spam-filtered mail can still appear as delivered when it was placed in Junk or quarantine.
Mail-flow rules and DLP actions can also appear in trace details.
Correlate outward
If a third-party gateway sits before or after Microsoft 365, message trace only tells you Microsoft’s part of the story. Match timestamps and identifiers with the other system.