S/MIME Mixed Security State when sending mail to one recipient

An user is sending an encrypted mail to one recipient. But the recipient gateway says "Mixed Security State - partially encrpyted". The recipient gets a mail with two attachments: txt and html. When he uses mail on the iphone instead of Outlook 2016 everything works fine. Also when he is answering the mail.

Some facts:

- User A sends a encrpyted mail to B by Outlook - Error Mixed Security State

- User B send a encrpyted mail to A and A answers this mail to B - OK

- User A send a encrypted mail to B by mail on the iPhone - OK 

Any ideas? Thx for your support!