Outlook 2007 Multiple Send And Receive Actions Until Locks Up

A customer of mine has a problem with Outlook 2007 SP1 + latest updates
running on Vista SP1 + latest updates whereby when he does a send and receive
and there is an email in the Outbox waiting to be sent, instead of carrying
out three send and three receive actions (one for each email account present)
Outlook starts doing many send and receive actions and finally locks up.

Relevant background info as follows:

I set up and installled the computer from new on 18th March 2009 and as part
of the installation set up and tested the email in Outlook 2007. I also
installed the Windows Mobile Device Center and paired the customer's HP iPAQ
running Windows Mobile 2003 SE to sync Contacts and Calendar with Outlook
2007. The email problem started mid-morning the next day (i.e. 19th March)
and the customer had used his email successfully a significant number of
times before this began to happen.

The customer has told me he hasn't changed anything that could cause this
problem. Over the telephone I have talked the customer through disconnecting
the HP iPAQ and deleting and recreating the email accounts but the problem
remains.

I have an appointment with the customer at 16:00 Monday 23rd March (GMT) to
investigate this problem and would appreciate any advice on how to
troubleshoot this issue. I would be particularly interested to hear if anyone
knows of any issues between Windows Mobile 2003 SE, WMDC and Outlook 2007.