Not all Outlook add-ins are programmed to cooperate with, or share internal Outlook objects. Conflicts usually present themselves as certain add-in features not working, but may also result in Outlook crashing. If your message keeps getting stuck in the Outbox, the problem may be linked to other Outlook add-ins which use the Outlook Object Model to access the message and inadvertently mark it as read. As Outlook won’t send read messages, it remains in the Outbox until you open the message and resend it. Add-ins that read or access your email as you are sending it are known to potentially cause this issue, such as Business Contact Manager (BCM), Norton Anti-Spam Outlook Plug-in, MS CRM 3.0 Add-in, iTunes Outlook Add-in, Salesforce.com Outlook Add-in.
You may disable these add-ins or try to install newer versions of these add-ins to prevent the issue. The best method to diagnose a conflict is to disable all add-ins, and then enable the add-ins one at a time (just those you actually use) until the conflict presents itself or is resolved. It’s always best to uninstall any add-ins you are not using.