Mail Apparently "sent" But Not Really

Exploding Vole

I am nearing my wit's end.

I'm running Mac Mail 1.3.11 (v622) on an iBook 6G4 (10.3.9). Have been delighted until two weeks ago, when my e-mails appeared to stop reaching their destination, though Mac Mail had dutifully stored them in my "sent" folder.

I contacted my ISP; they told me to send them a test e-mail via their website. That worked okay; trying to do the same via Mac Mail did not.

I have checked, re-checked, and re-re-checked my account settings. I even deleted them all and re-created them. In the advanceed options under "Accounts" I have clicked the "remove now" button for each of my accounts. No change. I receive e-mails fine. I just can't send them, no matter what Mac Mail says has happened to them. My ISP says they'd be bounced if there was something wrong at their end. No blockages at their end.

Desperate for a fix, I changed my user name on one of the accounts, to see where Mac Mail would store them. Sure enough, it placed it in my "sent" folder - then told me the user name was wrong. But - and I still can't fathom this - THE E-MAIL REACHED ITS DESTINATION.

What on earth is going on? Please help me before I turn to suicidal thoughts.
The problem is usually in the account set up in preferences. The last time a client had this problem the return address was invalid. It made no sense why fixing that corrected the problem you are having, but check it.

Also, you might try deleting the preferences plist file, but I am not as sure about that.