When such a user sends a mail to Discourse, they are rejected with Email::Receiver::InactiveUserError.
What’s the best method to prevent this issue? Would replacing the mail address of the inactive accounts work?
The InactiveUserError happens only when the user is not activated and not staged. If you want your users to be able to send emails in, you will have to either activate their account or mark them as staged.
I had a non-active, staged user (that had been disabled via the interface and staged via Rails). After the user logged in (via SSO), he had to confirm has address, became active and could log in, but is still staged: