"This account already exists" - Email

My Mac recently died. In doing a TimeMachine restore to a different machine, some things did not restore properly. (which was a real eye-opener and hugely disappointing). One item that was apparently mangled was the Keychain. This meant that most of my email passwords were "unknown" (who memorizes 14-character passwords?) I am working this as a separate issue with Mac Support - but the process has been VERY slow and I decided to reset the email accounts for which I have server access - since I can log into the server, change the password there then change the password on my email client to match.

For the most part - this has worked. I have most of the accounts working again.

One account is giving me headaches. When I go into Mail->Preferences->Accounts, or Mail->Accounts, it does NOT show up in the sidebar. But when I try to create the account, I get a message saying, "This account already exists". Frankly, I think the email setup process should be regarded as one gigantic "bug" - but until Apple produces a setup process that doesn't hamstring the end user, does anyone know a work-around for this? Is there some way to ensure the Mail setup for the account is REALLY deleted (so I can set it up again)? Or to make the account show up in the sidebar (so I can edit the account parameters)? Does anyone know where the mail account information is actually stored? I'm pretty comfortable on the command line, with regex, databases, shell scripting, etc. - but have to know where to start. Suggestions?

Thanks!