Outlook 2013 and 2010 issues sent items

I have 2 machines.

Laptop running Outlook 2013 and a PC running outlook 2010. Both connecting to the same Exchange 2010 account.

I send a email from the outlook 2013 account and it sends OK and gets placed in the sent items box. But in outlook 2010 it stays in the outbox.

The mail doesnt get resent it just gets stuck there. Any ideas??