Send Issues with Outlook for Mac (2011 / V15) and Exchange 2013 SP1

Question

Hi all,
we have a strange issue with only with Outlook for Mac Users (version 2011 and the newest Outlook Version (V15) from Office 365) connected to our Exchange Server 2013 SP1. Some emails which we send out to the exactly same recipient are not sent through our
Exchange Server. 
We have sent 8 emails with subject "test1, test2, test3...". Only test 3-6 was arriving to the recipient. The other emails are in the "sent" folder but it was not sent out through the Exchange.
What we can see with "Get-MessageTrackingLog" is, that the Recipients and MessageSubject are empty for these emails:
Timestamp Sender Recipients MessageSubject
25.02.2015 16:48:52 [email protected].. {}
25.02.2015 17:04:43 [email protected].. {}
25.02.2015 17:08:25 [email protected].. {[email protected]} test 3
25.02.2015 17:08:56 [email protected].. {[email protected]} test 4
25.02.2015 17:09:38 [email protected].. {[email protected]} test 5
25.02.2015 17:09:38 [email protected].. {[email protected]} test 6
25.02.2015 17:13:45 [email protected].. {}
25.02.2015 17:13:55 [email protected].. {}
We can still reproduce the issue on different Mac computers.
Thanks for any help in advance!
Thanks and best regards
Reto

Answer 1

Here is the mailheader form the successfully sent email:
Date: Wed, 25 Feb 2015 17:08:25 +0100
Subject: test 3
From: sender <[email protected]>
To: "'[email protected]'" <[email protected]>
This mailheader is from an email which was not sent out:
User-Agent: Microsoft-MacOutlook/15.6.0.150113
Date: Wed, 25 Feb 2015 16:48:52 +0100
Subject: test 1
From: sender <[email protected]>
To: [email protected] <[email protected]>
Interesting: Only the not received messages contains User-Agent string and have some quotation marks missing in the To field.