Most users typically use more than one computer &/device to connect to their mailbox. The issue described below is observed in Office 365 as well as an in-house hosted Exchange 2010 server.
In a nutshell: Emails sent from one computer (the main computer) & known to have been delivered show up on the other computer ( infrequently used , home computer) in the Outbox waiting to be delivered. In office 365 they show up in the Drafts.
That email need not be the last email sent. It could be any arbitrary email sent days or even weeks ago. Often there are 2- 3emails.Most of the other folders, such as sub folders, deleted items inbox etc. are completely in sync including the read /unread.
If I logon to the server via OWA, which I am presuming should be the definitive answer on the status of the email, the email is also in the Outbox. However the original computer shows the item as sent & we have confirmed by other means that the email
was delivered.
Note: We do not use PST’s. We do use OST’s . Have not been able to replicate on demand. We have to just wait for it to happen.