SBS 2011 / Exchange 2010 with pop3 connctor. SMTP server stops working

I've got a small system (4 users) running on SBS 2011. Over the past few months mail would stop flowing. We are using the pop3 connector to retrieve email from outside pop accounts.

If I receive a large message with attachments > 10mb The system will throw an error EVENTID 211 in the sbs log in event viewer. The sbs pop3 service and the transport service are still running, yet a "telnet localhost 25" will not yeild a connection. For some reason. A bad or large inbound message will cause the transport service to stop functioning and the only way I can restore it is to replace the C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\data directory with one from an old backup from before the problem began. I've tried many things, deleting all the pop users from the SBS console and re-creating them. Re-inputting all the receive connector settings etc. Nothing has worked, just restoring the directory. This now seems to happen several times per day and is causing a lot of problems.

I'm also getting some errors in the pop3service.log

163637-pop3service.txt

I notice that some times the transport service takes a very long time to start (or stop), but eventually does. Until I restore the data directory it will not allow a telnet localhost 25 connection and no mail will flow in or out.

How do I tract this down and solve? Many thanks.