spamtrap-milter is a sendmail milter intended to:

  * perform various lookups to confirm the incoming connection is intending
    to deliver spam or not.
  * reject any email that appears not to be spam, and is listed as in a
    domain file indicating what domains should not accept any email

The purpose of this is to allow the use of new spamtrap domains as
live spamtraps whilst at the same time conditioning the domain for non-robot
(i.e. legitimate) senders by rejecting all mail to any user for them.