SPAM or junk mail is usually sent by mailing a single message to a large number of (unwilling) recipients. As such, it usually does not contain the email address of all recipients (remember, junk mailers pay for these address lists). By rejecting messages that do not have the list address in the To: or Cc: header(s) a large fraction of spam to the list can be filtered out.
This filter function is activated by default, but will work only if you specify the list directory on the ezmlm-reject(1) command line. To disable this restriction, remove the ‘DIR’ argument from the ezmlm-reject(1) command line, or add the -T switch.
By default, this error is logged, and an error message is sent to the sender. Since virtually all the failures will be SPAM and virtually all spam has a faked SENDER, most of these error messages will go to the postmaster. Thus, you may want to use the ezmlm-reject -q switch (quiet) to suppress the sender notification.