After changing ISPs the former email account was removed, but our iPad still tries to log into it.

If we try to use the web interface it says the account is unknown. In the iPad Settings it says there's a problem logging in and asks to re-enter the password. In the meantime, all the controls for the account are greyed out until we successfully log in, which of course we can't.

I do not want to *remove* this account because it has years of archived emails. I simply want to turn it off. Can anyone offer a suggestion on how to do this?