Removing an account from Thunderbird doesn't mean you have to kill off or stop using the account and its address.

What are you trying to "reset"? What procedure are you following that involves entering the address and its password? That sounds like you're trying to add the same account again. Why?

Right, if the account uses IMAP then it saves its messages on a remote server. So you could remove the account and not lose anything because it would all be there waiting for you when you re-install the account in Thunderbird.

If it's a POP-connected account then you don't want to delete it, as you'd lose all the messages stored in it. In your position, I'd move all the messages that need to be kept to Local Folders, then delete the account and start over. And your proposed "restart without any leftover settings" would mean deleting the account (and much else).

So, first of all, is it POP or IMAP? If IMAP you can just delete and re-install the account. If POP, you need to set about rescuing its contents.

For a clean "as if just installed" re-start, you could delete your profile, or at least start over with a new one. But this would wipe all your accounts, address book, add-ons, customizations and so on.

https://support.mozilla.org/en-US/questions/thunderbird

It's set to IMAP. How do I delete the account?

Thanks for your help. Ron

Select the account. Go to Tools|Account Settings. The Account Settings page will appear. At the bottom there is a button labelled Account Actions. This will reveal a small menu including an entry to Remove Account.