Outlook sends following mail via EAS:

From: "xx XX" <nnn@xxx.yyy.zzz>
To: <mmm@kkk.xx>
Subject: einladung
Date: Fri, 6 Mar 2015 20:35:19 +0100
Message-ID: <012701d05844$ae1fe290$0a5fa7b0$@xxx.yyy.zzz>
MIME-Version: 1.0
Content-Type: text/calendar; method=REQUEST;
charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Mailer: Microsoft Outlook 15.0
Thread-Index: AdBYRKYVy92arwhaQ5iTBsYkRh+97AAAAeVg
Content-Language: de

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 15.0 MIMEDIR//EN
VERSION:2.0
METHOD:REQUEST
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
ATTENDEE;CN=mmm@kkk.xx;RSVP=TRUE:mailto:mmm@kkk.xx
CLASS:PUBLIC
CREATED:20150306T193518Z
DESCRIPTION:\n\n
DTEND;VALUE=DATE:20150308
DTSTAMP:20150306T193518Z
DTSTART;VALUE=DATE:20150307
LAST-MODIFIED:20150306T193518Z
ORGANIZER;CN="xx XX":mailto:nnn@xxx.yyy.zzz
PRIORITY:5
SEQUENCE:0
SUMMARY;LANGUAGE=de:einladung
TRANSP:OPAQUE
UID:040000008200E00074C5B7101A82E008000000001053700F4D58D001000000000000000
0100000009F6E7D06A7B0934B8FE97C642C849F96
X-MICROSOFT-CDO-BUSYSTATUS:TENTATIVE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INTENDEDSTATUS:FREE
X-MICROSOFT-DISALLOW-COUNTER:FALSE
X-MS-OLK-AUTOSTARTCHECK:FALSE
X-MS-OLK-CONFTYPE:0
END:VEVENT
END:VCALENDAR

It seems that sogo re-encodes the mail into quoted-printable. When the accepting the invite (from the received email) via the web ui following error message is reported in sogo.log:

Mar 06 20:33:18 sogod [30356]: [ERROR] <0xB70E2F38[SOGoMailObject]:1685> body part index out of bounds(idx=1 vs count=0): {bodyId = ""; description = ""; disposition = {}; encoding = "quoted-printable"; languages = (de); lines = 31; location = ""; md5 = ""; parameterList = {charset = "utf-8"; method = REQUEST; }; size = 870; subtype = calendar; type = text; }

The problem is that sogo fails to decode (quoted-printable) the invite before saving it to the calendar.

I hope this helps to fix the issue.

https://github.com/inverse-inc/sogo/commit/4826f0155552d56a98712dff1a607a2504b98591