I was reviewing mailboxes in System Manager looking for oversized mailboxes when I noticed that a couple of mailboxes had a username in "Last Logged On By' other than the name of the user. Does this mean that somebody else in the company is logging into their email or is there another possible explanation?

If the name is one of your other users, it may be because that user tried to access the other user's calendar. I am pretty sure that Exchange System Manager is showing you "access attempts", though and not actual accesses. It shows the last person to logon or try to logon to a particular mailbox. You can enable diagnostics logging for private information store logons also, and audit mailbox access that way. This type of logon could be someone opening another user's calendar through Outlook, for example. If it is 2 people that don't work together or it is people that should not be trying to open the boss' calendar, then you should probably investigate.

That's exactly what it turned out to be. Thanks for the insight.

You are welcome. Any time!