I'm not sure if this is the correct forum for this, as it does have something to do with the GWIA (kind of) but I'm not sure if the GWIa should be responsible for dealing with the situation.
When GroupWise sends out an iCal file, it uses the userid in the email address instead of the preferred internet address. While this typically isn't an issue, we are having issues with some other email services adding the address in the iCal file as another attendee and not showing the original address (the preferred internet address) as having accepted the meeting. This causes further issue if the users of those other email systems modify the appointments, as it ends up removing the appointment from our GroupWise calendars.
For a more visual understanding:
Userid: mhk1234
Preferred Internet Address: Mark.Kendell@abccompany.com
Source of email sent when accepting appointment sent by outsideuser@othercompany.com
Code:
Received: from mailserver (unknown [127.0.0.1])
	by mailgateway with ESMTP id 165EB6FC159
	for <outsideuser@othercompany.org>; Tue, 15 May 2012 11:43:42 -0400 (EDT)
Received: from mailserver ([127.0.0.1]) by mailgateway with ESMTP id EiWHxRmVCUhQMQA8 for <outsideuser@othercompany.org>; Tue, 15 May 2012 11:43:42 -0400 (EDT)
Received: from MTA by mailserver
	with Novell_GroupWise; Tue, 15 May 2012 11:43:41 -0400
Message-Id: <4FB2416A0200008D000678F7@mailserver>
X-Mailer: Novell GroupWise Internet Agent 8.0.2 
Content-class: urn:content-classes:calendarmessage
Date: Tue, 15 May 2012 11:43:38 -0400
From: "Carol Heinicke" <Mark.Kendall@abccompany.com>
To: "Carol A Heinicke" <outsideuser@othercompany.org>
X-ASG-Orig-Subj: Accepted: test2
Subject: Accepted: test2
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="=__Part5678A6BA.0__="

This is a MIME message. If you are reading this text, you may want to 
consider changing to a mail reader or gateway that understands how to 
properly handle MIME multipart messages.

--=__Part5678A6BA.0__=
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: quoted-printable

Item Type:  Appointment
Start Date:  Thursday, 17 May 2012, 08:00:00am (EDT)
Duration:  1 Hour
Place:  here
Action:  Accepted

This electronic message is intended to be for the named recipient, and may =
contain information that is confidential or privileged. If you are not the =
intended recipient, you are hereby notified that any disclosure, copying, d=
istribution or use of the contents of this message is strictly prohibited. =
If you have received this message in error, or are not the named recipient,=
 please notify us immediately by contacting the sender at the electronic ma=
il address noted above, and delete and destroy all copies of this message. =
Thank you.

--=__Part5678A6BA.0__=
Content-class: urn:content-classes:calendarmessage
Content-Type: text/calendar; method=REPLY; name=meeting.ics; charset=UTF-8
Content-Transfer-Encoding: 8bit

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Novell Inc//Groupwise 8.0.2 
METHOD:REPLY
BEGIN:VTIMEZONE
TZID:(GMT-0500) EST
BEGIN:STANDARD
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
DTSTART:20001104T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=1SU;BYMONTH=11
TZNAME:EST
END:STANDARD
BEGIN:DAYLIGHT
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
DTSTART:20000311T020000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=2SU;BYMONTH=3
TZNAME:Daylight Savings Time
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
DTSTART;TZID="(GMT-0500) EST":20120517T080000
DTSTAMP:20120515T154338Z
ATTENDEE;CN="Mark Kendall";PARTSTAT=ACCEPTED:
 MAILTO:mhk1234@abccompany.com
ORGANIZER;CN="Some Guy";ROLE=CHAIR:
 MAILTO:outsideuser@othercompany.org
SUMMARY:test2
LOCATION:here
DTEND;TZID="(GMT-0500) EST":20120517T090000
UID:52fac24d-024e-4079-ac80-298c99a7a2db
PRIORITY:5
CLASS:PUBLIC
X-GWCLASS:NORMAL
END:VEVENT
END:VCALENDAR

--=__Part5678A6BA.0__=--
So, is there something we need to change so that iCal files have the correct addresses in them or is this fixed in GroupWise 2012?