There have been reports of Outlook choking on email that Norton has detected something within. There have also been reports that Outlook has choked on large email and malformed headers.
In most instances using web based email access and removing the offending email allows Outlook to continue on it's merry way.
There is also a Utility that can be downloaded and installed for this purpose:
In every instance it has been found to be a specific problem with the email client and/or anti-virus software.
Hope this helps,