yup, disabling SMTP doesn't disable everything related to it.
So it's impossible to register via a invite, impossible to change a password, etc.
It would be really nice if there were just an option in the admin panel to hide the “email” field, or if it were simply disabled by default if the instance doesn't have SMTP.
The only viable solution to this problem right now is to sign up through the main menu (not via a prompt) and have the user set up two-factor authentication; this bypasses the email verification when the IP address changes.
But it probably doesn't work for password recovery.
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Comment by @Pseudal