PGP/MIME from PGP Desktop to Enigmail

I am currenty evaluating PGP Desktop (10.3.2 MP3) and I am trying to send PGP/MIME Mails from Outook 2013 using an Exchange Server to an Enigmail user.

The user receives the encrypted content, but Enigmail does not trigger.

All PGP Keys are good/avaible because I can decrypt the Message.pgp attachment manually.

I have the suspicion that the reason is a wrong content-type header.

The content-type contains

Content-Type: multipart/mixed;
	boundary="_003_cf2fb98a8699483f933eb93c088dc293DVWCDEVSRVTESTg"

which is not RFC3156 compliant.

Is there anything I can do or is this a limitation of PGP Desktop, that it cannot send RFC 3156 compliant mails?

According to the RFC the Content-Type header should look like this

Content-Type: multipart/encrypted; boundary=foo;
         protocol="application/pgp-encrypted"

Thanks