We’re seeing a resurgence of a problem with email getting marked as read and not sending. This was a problem several years ago, with most offending add-ins updated to prevent problems.
Users are reporting AVG 2011 is causing problems sending mail. The fix is to disable the avgoutlook.Addin.
For more information and instructions, see
After viewing the Outbox, the messages in it won't send
After viewing the Outbox, messages won't send
Published October 19, 2010. Last updated on September 17, 2011.