It's a new year so my ISP issues new certificates, fine on MacOS I just tell it to trust them, but on iOS I have no control. All I can do is press "cancel" when the pop up (spam?) warning message appears, and wait 2 seconds for the same message, cancel > popup > cancel > popup. I can't quit mail, launch springboards or do anything else when this loop happens. My only option was to turn off the phone and delete the account.