SBS 2011, Exchange Server queue

Hi.

We have an SBS 2011 server running exchange. All of a sudden the Submission email queue is getting up to 50 000 emails in it (neither recipient or sender of these emails are accounts of this exchane server). These emails disappear after a while, then if I restart the transport service the emails build up again.

I also have 17 000 emails in a queue under "yahoo.com" that dont seem to go away, eve after selecting remove without NDR.

What could be going on here? I dont think the server is an open relay.

Thanks.