problem with iphones and calendar

Hi,

i've exchange 2013 with cu11 and i have problems that all iphones (not specific iOS) can't send calendar outside the organization.

this is a fresh install with windows 2012 R2.

please help..

this is the error that i get back from the exchange

Remote Server returned '550 5.6.0 M2MCVT.StorageError.Exception: ConversionFailedException, Content conversion internal failure.; storage error in content conversion.'

‏‏

Received: from ex server name (ip) by

 ex server name (ip) with Microsoft SMTP Server

 (TLS) id 15.0.1156.6; Tue, 17 May 2016 14:25:06 +0300

Received: from ex server name ([ip]) by

 EX server name ([ip]) with mapi id

 15.00.1156.000; Tue, 17 May 2016 14:25:06 +0300

Content-Type: application/ms-tnef; name="winmail.dat"

Content-Transfer-Encoding: binary

From: Shikma Turgeman -  <email>

To: Avi Grinberg <email>

Subject: =?iso-8859-8-i?B?8Onx6eXvIODn+OXvIA==?=

Thread-Topic: =?iso-8859-8-i?B?8Onx6eXvIODn+OXvIA==?=

Thread-Index: AQHRsC7DHZ1OXEDF0UWP7bMW2QH7wg==

Date: Tue, 17 May 2016 14:25:05 +0300

Message-ID: <17F0D6FD-83E6-4B86-8242-EDA9B570916A@domain>

Accept-Language: he-IL, en-US

Content-Language: he-IL

X-MS-Has-Attach: yes

X-MS-TNEF-Correlator: <17F0D6FD-83E6-4B86-8242-EDA9B570916A@bdomain>

MIME-Version: 1.0

X-MS-Exchange-Transport-FromEntityHeader: Hosted