BounceHandler un-confirms email addresses which become invalid (mailbox full, email address deleted, etc), but a part of the bounces are email addresses which do not exist at all because of a typo. In this case the person could be unaware his/her email address is not confirmed (since s/he has not received the confirmation email) and s/he is no more able to recover her/his password in case of loss (amongst other things).

I propose to add a notification (Echo) when the confirmation status is “unconfirmed” and when a bounce is received with a permanent status of failure of type "Unknown user"/"No such user", in order to warn the user (on a working channel since the email is no more functional) that his/her email address is nonexistent.

This task has links with T152147 and T58074. In particular, it should be coordinated with the latter to avoid sending both notifications that the email address is nonexistent and should be confirmed.