Welcome to the forums! I see you've decided to make your first post a good one
Can you also post the Event ID from the Application log?
Does this happen with other users?
Is this person above any quota limit you have set?
Are there other messages similiar to this error but not exactly the same? Usually a series of messages in a row will be generated for a single problem.
Can you move this user to another mailbox store and then back to the original store?