I’m pretty sure that nobody actually uses mailbox protocol yet.
It’s conversion from NCQv1 -> NCQv3 may be tricky and requires some hand working. So if (just in case) you used to use Mailbox protocol – contact me if you need assistance to fix it.
( dzuikov@gmail.com | @voidlizard (telegram ) )
Deleting mailboxes does not solve the problem.
Note, that this protocol is VERY experimental and not it is not tested properly yet and most probable requires more development.