If you view the Outbox while messages are waiting to be sent, the messages are marked as read and won't send.

This is caused by a Net framework add-in you have installed that 'reads' Outlook items, including the Send to OneNote addin, BCM (Business Contact Manager), and more recently, the iCloud add-in installed by iTunes. A list of other add-ins known to cause this behavior is listed below.

After Installing iCloud, Mail Won't Send? | Messages are stuck in the Outbox

Antivirus/security software is often the culprit as it is looking for viruses and spam. Disable add-ins installed by the security software to solve.

iCloud is quickly passing Anti-virus add-ins as a frequent cause. It's installed by iTunes and if you are not using syncing with an Apple device using iCloud, you can disable it. Apple also installs two additional add-ins (Change Notifier and Outlook Add-in) that are not needed if you don't sync with an Apple device. If you are using iCloud, avoid viewing the Outbox and switch to another folder before re-sending messages you opened from the Outbox. See Outlook and iCloud 2: What you need to know for the more information.

To send the affected messages, you'll need to open the message, then switch to a different folder before clicking Send on the message. Do not go open the Outbox until after the messages are sent.

To identify the add-in that results in this behavior:

In Outlook 2010 though current versions, look in File tab, Options, Add-ins. Disable the add-ins and re-enable them one at a time and test. In Outlook 2007 Look in Tools, Trust Center, Add-ins.

Click Go button at the bottom of the dialog to disable add-ins. Select Disabled Add-ins from the dropdown menu to re-enable add-ins.

Since BCM and possibly other add-ins are not listed in the COM add-in list, check
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins

HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Addins

If using Windows 64-bit, addins are in the following HKEY_LOCAL _MACHINE key.

HKEY_LOCAL_MACHINE\Software\WOW6432Node\Microsoft\Office\Outlook\AddIns

When you use a 64-bit OS, some add-ins are listed under both HKEY_LOCAL_MACHINE locations. Check the HKEY_LOCAL _MACHINE\Software\WOW6432Node path first but if its listed in both locations, you may need to set LoadBehavior to 0 in both locations.

Export the keys then change the LoadBehavior value to 0 to disconnect an add-in. Restart Outlook and test.

If you need a quick tutorial on editing the registry, see How to Edit the Registry

Applications responsible

The following is a list of some of the applications which are reported to cause message to be marked as read. If you use these or any other add-ins, check for a newer version or an update - newer versions of some of these applications will not cause problems. If the problem is caused by the newest version, disable or uninstall the addin.

• iCloud addin. used to sync with Apple devices. Disable if you are not using it to sync.
• AVG (Free version tested) - disable avgoutlook.Addin. Set LoadBehavior 0. (From "Thunderbird": you need to disable two keys for avgoutlook.Addin when using 64-bit Windows 7)
• Bitdefender Internet Security
• Symantec Endpoint Protection
• Social Connector with Outlook 2007
• Nuance PDF Outlook Add-in

If you are using one of these older applications, look for a newer version:

• Evernote Clipper
• Business Contact Manager (BCM)
• Send to OneNote from Outlook PowerToy
• Outlock
• Outlook Backup Add-in
• Sierra Wireless Outlook Integration - disable in the registry. It's listed at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\ SwiOutlookInt.Addin
• Timeslips add-in
• Attensa for Outlook. Check for upgrade: newer versions not affected.
• Cisco Meetingplace plug-in
• MindJet Mind Manager 6. Disable the add-in in the registry - the key is Mindjet.Mm6OutlookLinker.AddIn.2
• ScanSoft PDF Converter Outlook Addin
• Norton AntiSpam Outlook Plugin
• Norton Internet Security
• Salesforce
• Upshot Outlook integration
• Xobni - Upgrade to the newest version of Xobni.
• Auto-Mate
• RingCentral Add-in
• DesktopSMS.Addin
• ABBYY PDF Addin