Here it is, I've replaced most of the real users and domains with "user" and "domain", the important line with the "local" part is;
ORGANIZER;CN="user":mailto:user@domain.local
Zimbra seems to use this "local" domain as the reply address when accept is clicked.
Full headers below;
Return-Path:
user@domain.co.uk
Received: from server.doamin.com (LHLO
server.domain.com) (10.0.0.2) by server.domain.com with
LMTP; Mon, 3 Nov 2008 15:36:01 +0000 (GMT)
Received: from server.domain.com (server.domain.com [10.0.0.2])
by server.domain.com (Postfix) with ESMTP id 8BFC1CCBEC5;
Mon, 3 Nov 2008 15:36:01 +0000 (GMT)
Received: from [212.57.240.49] (helo=mail.domain.co.uk)
by server.domain.com with esmtp (Exim 4.63)
(envelope-from <user@domain.co.uk>)
id 1Kx1TE-0000Cq-Pl
for
mike@domain.com; Mon, 03 Nov 2008 15:36:01 +0000
Received: from [192.168.100.52] (helo=oldfieldpark)
by mail.domain.co.uk with esmtp (Exim 4.66)
(envelope-from <user@domain.co.uk>)
id 1Kx1VR-0005jc-Px; Mon, 03 Nov 2008 15:38:18 +0000
From: "user" <user@domain.co.uk>
Sender: "user" <user@domain.co.uk>
To: <user@xyz.co.uk>,
<mike@domain.com>
Subject: Tyres
Date: Mon, 3 Nov 2008 15:38:37 -0000
Message-ID: <00a201c93dca$3d5ccd70$b8166850$@xyz@domain.loca l>
MIME-Version: 1.0
Content-Type: text/calendar; method=REQUEST;
charset="utf-8"
Content-Transfer-Encoding: 7bit
X-Mailer: Microsoft Office Outlook 12.0
thread-index: Ack9xNr4yDyop8jWQYeQDjciOVqAzgABWGmw
Content-Language: en-us
BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 12.0 MIMEDIR//EN
VERSION:2.0
METHOD:REQUEST
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
ATTENDEE;CN='user@blahblah.co.uk';RSVP=TRUE:mailto :
user@blahblah.co.uk
ATTENDEE;CN='mike@domain.com';RSVP=TRUE:mailto:mik e@domain.com
CLASS:PUBLIC
CREATED:20081103T153836Z
DESCRIPTION:When: 04 November 2008 16:30-17:00 (GMT) Greenwich Mean Time :
Dublin\, Edinburgh\, Lisbon\, London.\n\n*~*~*~*~*~*~*~*~*~*\n\n\n
DTEND:20081104T170000Z
DTSTAMP:20081103T153836Z
DTSTART:20081104T163000Z
LAST-MODIFIED:20081103T153836Z
ORGANIZER;CN="user":mailto:user@domain.local
PRIORITY:5
SEQUENCE:1
SUMMARY;LANGUAGE=en-us:Tyres
TRANSP:OPAQUE
UID:040000008200E00074C5B7101A82E00800000000C0E9F8 DAC43DC901000000000000000
010000000BCF4BA90E13ADE4D94EC41B92E717385
X-ALT-DESC;FMTTYPE=text/html:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//E
N">\n<HTML>\n<HEAD>\n<META NAME="Generator" CONTENT="MS Exchange Server ve
rsion 08.00.0681.000">\n<TITLE></TITLE>\n</HEAD>\n<BODY>\n<!-- Converted f
rom text/rtf format -->\n\n<P DIR=LTR><SPAN LANG="en-gb"><FONT FACE="Calib
ri">When: 04 November 2008 16:30-17:00 (GMT) Greenwich Mean Time : Dublin\
, Edinburgh\, Lisbon\, London.</FONT></SPAN></P>\n\n<P DIR=LTR><SPAN LANG=
"en-gb"><FONT FACE="Calibri">*~*~*~*~*~*~*~*~*~*</FONT></SPAN></P>\n\n<P D
IR=LTR><SPAN LANG="en-gb"></SPAN><SPAN LANG="en-gb"></SPAN><SPAN LANG="en-
gb"></SPAN></P>\n\n</BODY>\n</HTML>
X-MICROSOFT-CDO-BUSYSTATUS:TENTATIVE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-ALLOWEXTERNCHECK:TRUE
X-MS-OLK-APPTSEQTIME:20081103T150118Z
X-MS-OLK-AUTOSTARTCHECK:FALSE
X-MS-OLK-CONFTYPE:0
X-MS-OLK-SENDER;CN="user":mailto:user@domain.co.uk
BEGIN:VALARM
TRIGGER:-PT15M
ACTION
ISPLAY
DESCRIPTION:Reminder
END:VALARM
END:VEVENT
END:VCALENDAR
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.local domain in appointment request 
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