Item in Outbox being archived

We are doing a migration from Exchange 2007 to 2010.

We are finding that items in the Outbox are being archived, I've honesty never come across this before. Is this expected behavior? Will EV archive the Outbox just like any other folder?

I assume so and if this is happening at a higher rate after a mailbox has been moved because items are staying in the Outbox for a longer period of time?

 Any advice/guidance appreciated.