Evite's mail headers are clever: they set the envelope-from to the human dupe on whose behalf they are sending the mail, but the mails themselves originate at citysearch.com:

Return-Path: <address-of-human@wherever>
 ...
Received: from lax1evtmx7.citysearch.com (lax1evtmx7.citysearch.com [209.104.61.33])
   by [...my local server...] (envelope-from address-of-human@wherever)
Received: (qmail 15737 invoked from network); 16 Aug 2003 01:57:14 -0000
Received: from lax1evtwww14.prod.tmcs (HELO lax1evtmx5.citysearch.com) (192.168.61.114)
    by 0 with SMTP; 16 Aug 2003 01:57:14 -0000
X-Sender: info@evite.com
From: <info@evite.com>
Reply-To: <address-of-human@wherever>

I tried adding this to /etc/mail/access:

citysearch.com   550 Spamming denied
evite.com        550 Spamming denied

(Note: Suggestions that involve procmail, or that involve installing any mailer other than sendmail 8, will be gleefully ignored, so please don't waste your time.)

>Update, 7-Sep-2003:

The prevailing theory had been that I could do this, since Evite was always including X-Sender: info@evite.com:

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}

But now those cockgobblers have changed their X-Sender header to be the human sending the evite, making it useless for filtering. My next try is to filter on the Recieved headers and try and bounce anything that comes from citysearch:

LOCAL_RULESETS
HReceived: $>Check_Received
D{MPat}citysearch.com
D{MMsg}Spamming denied
SCheck_Received
R${MPat} $*$#error $: 553 ${MMsg}
RReceived: ${MPat} $*$#error $: 553 ${MMsg}

(There's a TAB after $*.)

(I can't just block based on IP addresses because my mail server is behind my ISP's relay host, so the hostile mailers never connect to me directly.)