Every time I check email, I get Unknown Error has occurred in Outlook.

Error Log says - A message in your Outbox could not be sent. The account which created the message cannot be found. There is nothing in my Drafts folder, nothing in my outbox, nothing on my computer. It could be from an account that was deleted. How do I get rid of this annoying error message? I don't really care about the unsent message. 

Appreciate help.