Content-Transfer-Encoding Header missing?

Hello,

our Exchange 2013 CU 6 won't send "Content-Transfer-Encoding" Header when sending Mails to external Domains. (This Header is present if Mail stays within our Org)

Already tried to set different Encodings - but without luck ... Exchange just won't generate this Header ...

[PS] C:\Windows\system32>Set-RemoteDomain -Identity Default -ByteEncoderTypeFor7BitCharsets Use7Bit
[PS] C:\Windows\system32>Set-RemoteDomain -Identity Default -ByteEncoderTypeFor7BitCharsets UseBase64
[PS] C:\Windows\system32>Set-RemoteDomain -Identity Default -ByteEncoderTypeFor7BitCharsets UseQP

Hope someone here can help me with this!?

Thank you, bye from Austria
Andreas Schnederle-Wagner