Quite to the end of Jaguar installation you have to set your email address, POP address, and SMTP servers. But the installer doesn't allow yout to set a protected (for instance, with a password) SMTP; it just allows to indicate SMTP name. And when you launch Mail and set the password, you then have TWO SMTP servers with the same name. The first one was created during Jaguar installation, and the second one when you've entered the password. To delete the first of them, you have to:

1. Install the Developer Tools
2. Open ~/Library/Preferences folder
3. Click on "com.apple.mail.plist" while holding Control and select "Property List Editor" from the Open With menu (it should appear as the default app)
4. In the "DeliveryAccounts" subtree select "0" (zero) and click on "Delete".
5. Save your changes and quit the editor.

That's all.

[robg adds: I haven't tested this, but you should also be able to edit the file in a pure text editor and remove the first entry <dict> entry in the section that begins:

<key>DeliveryAccounts</key>

As with most modifications, you should make a backup of the file before you start chaning things.]