users whose email fails temporarily are stuck. we should detect if it's working again
when a user's email is not working well, we detect it and mark it as bouncing.
then we tell the users that they must change their email when they try to login.
so, the user has to change their email, however there is a bug with that, #1620, so they are stuck.
besides fixing #1620, we should also attempt to detect if their email is working again, for instance, the next time they try to login (after a while?).
if it is, then we can mark it as not bouncing any more.
"detect" is tricky, just checking MX may not be enough. we really should send them a test email with a new validation link as well.
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