Mail keeps asking for account password when trying to send via Yahoo

• 2020 M1 MBA running Monterey
• Multiple times a week, Mail.app asks for password when trying to send...
• ...then chews on the password for several minutes before coming back with the nonsensical error "This account already exists"...
• ...then 10-15min later, even with that error message still displayed, suddenly we hear the "Woosh!" and the message has been sent
• ...except once in a while, entering the password works immediately

• The password it wants is the app-specific password (login password produces an error immediately when entered under these circumstances)
• Connection Doctor chews and chews and chews on the SMTP connection for about 20 minutes before finally producing a green light
• We have tried removing Mail.app's app-specific password at mail.yahoo.com and creating a new one — no dice
• This problem cannot be reproduced on other email apps
• There is no problem logging into mail.yahoo.com via browser

• This is only the latest problem we've had with Mail.app and Yahoo
  • Previously we had to switch from IMAP to POP to stop Mail from eating up GB/day of bandwidth by constantly syncing folders over and over and over and over
• And even though we've disabled the IMAP account, it still appears as one of the options for Outgoing Mail Server in Preferences > Accounts > Server Settings...
  • HOWEVER it's also impossible to get rid of:
  • When we go to Edit SMTP Server List, the IMAP account's STMP is not listed

Would love some knowledgable suggestions.