Mail stays in Outbox

Question

I know at least four people who are having this problem as well as myself and each of us has a different Internet host, but all of us started experiencing this problem when we upgraded to Tiger OSX 10.4.8. WE can receive mail just fine, but when we send an email, it just goes to the Out box and sits there. When we run connection doctor, we are told that our Macs are communicating with SMTP and other servers just fine. And we don't get any error messages. Just mail that sits in the outbox. We've all gone to our respective Internet providers and gone through the rigamarole of resetting from Port 21 to 547 and back again. Two of us have done a clean install of System 10. Still no dice. WE can all send mail just fine with Yahoo mail and to make matters even more mysterious, I can send mail from my laptop going through my same account and mail servers. It's just my desktop Mac that's having the problem (although my friend tried to login and send on his laptop through airport to our internet connection and was NOT able to send mail.)
Help
G5 Power Mac   Mac OS X (10.4.8)  

Answer 1

Mail silently (i.e. with no error message provided) going to the Outbox and sitting there would be typical of an account that's "offline", but I assume that's not the case, i.e. Mailbox > Go Online is disabled (grayed out), right?
I wonder whether this could be some form of index corruption. You may want to try the following and see whether it makes a difference:
1. In Mail, move any messages that may still be in Outbox out of there, to some other mailbox.
2. Quit Mail.
3. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong -- e.g. by dragging it to the Desktop while holding the Option (Alt) key down.
4. Locate Envelope Index and move it to the Trash. If you see an Envelope Index-journal file there, delete it as well.
5. In the Finder, go to ~/Library/Mail/Mailboxes/, locate Outbox.mbox and move it to the Trash.
6. Open Mail. It will tell you that your mail needs to be "imported". Click Continue and Mail will proceed to re-create Envelope Index -- Mail says it's "importing", but it just re-creates the index if the mailboxes are already in Mail 2.x format.
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder, i.e. ~/Library is the Library folder within the user's home folder.