The following describes how to filter incoming email messages based on the presence and contents of certain headers using sendmail v8.
This technique is directly copied from http://www.livejournal.com/users/jwz/245249.html?thread=1864449#t1864449, a discussion of how to bounce annoying messages from a particular service, but without the use of procmail or other client-level filters.
X-Sender: info@evite.com From: <info@evite.com>
If that's in every message, try this in your .m4 or .mc file:
LOCAL_RULESETS
HX-Sender: $>Check_XSender
D{MPat}info@evite.com
D{MMsg}Spamming denied
SCheck_XSender
R${MPat} $* $#error $: 553 ${MMsg}
RX-Sender: ${MPat} $* $#error $: 553 ${MMsg}
There are tabs betwen $* and $#.