Domino Access for Outlook blocks mail.box

I have a very strange problem. One of our users (the CEO) is running Domino Access for outlook as he does not like Notes. Normally I would force all users to use Lotus client or webmail but this was an exeption. The problem is that some of his mails blocks mail.box on our server. Suddenly I can see that there is 800 mails in queue and usually I paste theese mails to another server to route them. When I delete this users mail all mails are routed as usual. It only happens with some mails from him. I have not seen any specific about them like attachments etc.

Someone plese advise what to do…