What is the most likely reason why the message fails to appear in the message trace?

You have a Microsoft Exchange Online tenant that contains 1,000 users.

A user recently sent an email message that was never received by a recipient on the internet. From the Exchange admin center, you successfully run a message trace but cannot see the message in the trace.

What is the most likely reason why the message fails to appear in the message trace?
A . The user addressed the message to multiple internal and external receipts
B. The user encrypted the email message.
C. Your administrative user account requires additional rights.
D. The message is in the user’s Outbox folder.

Answer: D