Changing from one internet server to another

I'm using an iMac G4 on OS X 10.3.9 (I know, OLD!). I recently switched internet providers and got some help setting up the new account, wireless modem, etc. I use the Mac Mail program that came with the computer for email, but do not use .Mac as a provider.

The new system has been working just fine for over a month, but yesterday the old account was finally turned off. Now, I can't send mail. I have gone into preferences on the Mail program and deleted the old account. It no longer shows up on the list of accounts anywhere in the Mail Program, that I can find. But, when I try to send a new email, I get the following error msg.

"Enter password for Account "mail-dw.dmv.com [old email address]

The SMPT server "mail-dw.dmv.com" rejected the password for user: [old email address]. Please reenter your password or cancel."

Click to expand...

I have doubled checked that, in Mail Preferences, all server information is accurate, that the old server was deleted as well as the old account. So far, nothing is helping. The computer is acting as if it still has the old server information stored somewhere, but I can't think where else to look.
Hopefully, someone here can tell me where to look for that old account setting so that I can delete it and move on.

Thanks!

Related thought:

If all else fails, would it be wise or possible to consider reinstalling Mail from my original Software disc? If so, would there be any risk to Address Book (assuming not)?

Why don't you create another user account on your Mac. In that login, try setting up the Mail app. You'll be doing so without vestiges of your previous setup. If you can get it working in the new account, make sure you have it setup EXACTLY the same way in your usual account.