You'll probably find yourself using the message tracking logs to find if an email has gone missing. But what should a normal working mail transaction look like? Well below shows it, where the mailboxes are stored on the same server.

This is a message going from fred@sender.com to bob@receive.com. Basically if it says DELIVER STOREDRIVER, it means the message has gone into the mailbox, if it doesn't appear, it means there's some spam filtering or weird rules setup in the users Outlook that is putting it somewhere else!