Such a feature doesn't come with sendmail 8, however, it is available from others:
http://www.cs.niu.edu/~rickert/cf/ require_rdns is the one you are looking for.
Such a feature doesn't come with sendmail 8, however, it is available from others:
http://www.cs.niu.edu/~rickert/cf/ require_rdns is the one you are looking for.