(Sorry, not got many points left to give!)
The problem is a machine running Outlook which repeatedly downloads the same email, effectively blocking the account.
This only seems to happen with emails with large (4MB typical) attachments. Outlook is set to delete emails after 5 days but previously worked fine.
The offending email can be deleted in webmail but the problem happens again within a day or two.
I've tried increasing the timeout to no avail.
Any other ideas?