Earlier today I had the pleasure of troubleshooting what looked an awful lot like Norton Internet Security wrecking havoc with Outlook Express, but it just wasn’t so. The user was attempting to send mail, and was getting an error to the effect that SMTP was failing for an unknown reason on port 0. As we all know, SMTP runs by default on TCP port 25, or alternately on TCP port 587. For it to be operating on another port would normally indicate either a misconfigured mail client or a seriously misbehaving firewall or mail proxy.

Disabling Norton can be a bit of a hit-or-miss endeavor, as it is somewhat notorious for not actually leaving your traffic alone when you tell it to. We attempted all the usual Norton-bypassing tricks, nearly going so far as to uninstall it, when we discovered that the problem was OE itself. Error 0x800C0131, the cryptic Microsoft code for unknown errors with SMTP transactions, can also be caused by, of all things, a corrupt DBX file. The telling symptom showed up when, on a whim, we check the “Sent Items” folder, which wouldn’t load, complete with another cryptic error message. The solution follows:

1. Open Outlook Express
2. Select Tools » Options » Maintenance
3. Click the button labeled “Store Folder…”
4. Select and copy the path to the Store Folder ([CTRL] + [C] to copy)
5. Close out of all Outlook express windows
6. Go to Start » Run and paste the Store Folder’s path in ([CTRL]+[V] to paste)
7. Click the button labeled “OK” to open your Store Folder
8. Delete the file named “Sent Items.dbx”
9. Restart Outlook Express
10. QED

Here’s a relevant Microsoft KB article. A big hat-tip to mcse.ms for having a Google-searchable forum. How such a thing can be reliably troubleshot from a starting point of “I can’t send mail” remains a mystery.