This Zero-Mailer is even less than a nullmailer. It is a sendmail replacement which (basic configuration) send all mails to a single email address (ignoring the destination address).
It’s desigend for servers, which are no mail servers, and only sends logs, notices and such things. So all other software (may be scripts, used from the web-server) have to use a SMTP server for outgoing mail (e.g. forum).
It can be configred that when sendmail is invoked by a user, that all those mails are going to another address. (the users which calls sendmail is important, not the destination).