When DMARC configs of the email server do not allow MediaWiki to send emails on behalf of the user via Special:EmailUser (see T66795), these emails will bounce over and over again (see T134886). In such circumstances, BounceHandler should return the email to the sender and provide some information about why it bounced and what can be done.

Possibly, the returned email can be prefaced with something like "MediaWiki was unable to send this message, because it boucned. It appears that your email server may not allow MediaWiki to send emails on your behalf; please use a different email provider, or notify your wiki admins to enable the $wgUserEmailUseReplyTo option".