Outlook.exe task does not close

Issue:

You close Outlook and all office applications but the outlook.exe process is still running under processes.

Quick:

Disable antivirus add-ins, Tools => Options => Other tab => "Advanced Options..." => "Add-In Manager..." => Un-check any antivirus addins.

Visual:

Learning:
Anitivirus programs that scan Outlook email messages may keep Outlook from closing out completely. This will hold pst files open and keep files locked until the process is ended. You should always be careful not to open suspicious attachments anyway, so you should be able to disable the Outlook hook.