Hello from Prague to Japan and thank you for trying Rebex Secure Mail for your japanese c# email needs!
The transfer encoding of the other headers (including Subject) is automatically determined by the Rebex Secure Mail component.
Actually, the component chose QuotedPrintable encoding because the result was shorter ("=1B$B$3$l$O%F%9%H%a!=3C%k$G$9=1B=28B") than the Base64 encoded form ("GyRCJDMkbCRPJUYlOSVIJWEhPCVrJEckORsoQg==").
Do you think that it would be more suitable for Japanese emails to use base64 encoding for the headers and why?
If yes, please let us know and we could change the behavior of Rebex Secure Mail to automatically handle this.
P.S. actually, neither the
MimeMessage.TransferEncoding sets the transfer encoding of the headers like Subject, From, etc. In fact these properties only affect the email body. And it is useless to set both of them becuase both translate into the same "Content-Transfer-Encoding" header in the resulting MIME message.