Deactivate Account While Retaining Emails

Back in October Microsoft had us change Windows Essentials Email setting from a POP account to an IMAP account. Now I have two accounts so my email tries to periodically send from the old defunct POP account. I do not want to delete the old account because I would like to keep the contents on my PC as a backup. I have tried to delete the server URL and login in hopes to neutralize it, but it won’t let me. So is there a simple way to decommission my old account while retaining the emails from it? Thanks!