Please read all the symptoms. This problem did not start until Windows 7. I have the same problem on 3 PC's at the office.
Windows 7 Professional and Office 2007 Professional. I have removed the check from the "Send/Receive immediately when connected" box. This requires the additional Send/Receive click to get email out. Send/Receive works just fine. You can create email and they send and are moved to the Sent Items folder. However, if you look at the Outbox before the click on Send/Receive the email will not leave the Outlbox. The title in the Outbox is no longer bold. I have opened the email and tried to resend with no success. The title does not return to bold. I have exited Outlook and reopened. The Outbox still has the (1) beside it but the title is still not bold and the email will not send. It's as if the status of the email has changed and will not change back for sending. If I mark the email as unread the title will return to bold but the email still will not send.
Again, if you create an email it will send and be moved to the Sent Items folder but if you look at the Outbox the email will not send. I have to delete the email from the Outbox and recreate. If I don't look at the Outbox everything works.