Outlook 2003 doesn't recognize meeting invitations created and sent by sogo.
After some research and testing I was able to modify such an email in a way, that Outlook accepts it as working meeting request.

Sogo's invitation is something like that:
<snip>
------=_=-_OpenGroupware_org_NGMime-29475-1353425574.824758-3------
content-transfer-encoding: quoted-printable
content-type: text/calendar; method=REQUEST; charset="ISO-8859-1"

BEGIN:VCALENDAR
PRODID:-//Companio AG.//CalDAV Client//EN
VERSION:2.0
METHOD:REQUEST
BEGIN:VEVENT
DTSTART:20121212T080000Z
DTEND:20121212T090000Z
DTSTAMP:20121120T152523Z
UID:bd15749f-6078-47d5-b940-e25c23a70ea7
CREATED:20121120T152523Z
DESCRIPTION:hallo du hallo du
SUMMARY:neuer Termin
LAST-MODIFIED:20121120T153252Z
LOCATION:Phantasia
TRANSP:OPAQUE
ATTENDEE;PARTSTAT=3DNEEDS-ACTION;ROLE=3DREQ-PARTICIPANT;RSVP=3DTRUE;CN=3D=
xx
x@companio.de:MAILTO:xxx@companio.de
ATTENDEE;PARTSTAT=3DNEEDS-ACTION;ROLE=3DREQ-PARTICIPANT;RSVP=3DTRUE;CN=3D=
x
xx@cpdev.cpronto.org:MAILTO:xxx@cpdev.cpronto.org
ATTENDEE;PARTSTAT=3DNEEDS-ACTION;ROLE=3DREQ-PARTICIPANT;RSVP=3DTRUE;CN=3D=
sascha.zie
sel@companio.com:MAILTO:sascha.ziesel@companio.com
ORGANIZER;CN=3DCarlos Fortu:MAILTO:xxx@cpdev.cpronto.org
SEQUENCE:1
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
END:VCALENDAR

------=_=-_OpenGroupware_org_NGMime-29475-1353425574.824758-3--------
</snip>
The request shown above is not accept by Outlook as valid meeting request.

After adding that line:
Content-Class: urn:content-classes:calendarmessage
after content-type

everything is fine and the meeting is shown correctly within Outlook.

Is it possible to add that string during generation of meeting requests?