When someone creates an account, signs up for email newsletters and then their email address becomes invalid MKDoc continues to send out email that then bounce back.
There is no way in the admin or superuser interface to know that email is bouncing or to do anything about it other than delete users accounts.
The only way to deal with this at the moment is for the mail servers postmaster to keep track of the accounts that bounce and then to manually disable the sending of emails to these users at a MySQL level.
(TODO: add the MySQL commands needed here)