Sometimes it could happen that an inbound or outbound message didn't reach our mailbox or the recipient's inbox. This could happen due to different reasons, but you'll be able to check after the reason.
In order to discover it, you can check after the affected message in the Email Live Tracking section within the Control Panel, and then expand the "Info" section for this message:
Once within the message's details, you can check after the reason here:
After discovering the reason, you can check the description for all of the classification reasons here