Outlook.exe is hanging

I have an end user that is experiencing a problem with
Outlook. When he closes Outlook, the outlook.exe remains
in the process monitor (task manager). So even though he
has closed Outlook, when he tries to shut down, it pops up
an end task error for Outlook. I haven't been able to
find any information on the web regarding this problem.
Can anyone help me?

Thanks in advance!
Jenn