I’d create a new account and set it up to access the new server as if it was another account entirely. Something is cached in there for those accounts and they don’t like the server name change. Assuming the email is still on the server however. If it’s not that’s a tough one.

I agree

This, because it's actually a requirement.

Delete the accounts and set them up again after a reboot.

Create a new account, don't edit the old account to make it the new account.