Mail not allowing me to delete messages

This is for a Yahoo mail POP account that has worked fine for last 3+ years..
whenever I try to delete a msg, it goes away for a second and comes back as was, with the following error popup:

All other mailboxes are functioning fine.
A copy of the message I delete from this Yahoo acct, does go in the Trash folder but the original stays in Inbox.
I've emptied Trash, verified all settings, I don't remember changing anything, quitting the app and restarting my computer.
Its been days now since this problem first came up...any idea what's wrong?

 

tried both, still the same.