It appears that the windows 10 default app does not adhere to the RFC 5322 spec and respect the reply-to field correctly. We have a website that allows users to e-mail another user. The second user is then able to respond to directly e-mail the first user by clicking reply. The first e-mail is send by us and we use the message header to tell the program where the replies should go. However, we have had several instances of the Windows 10 Mail App replying to the from address and not the reply-to address.
I realize the high spoofing potential if users are not vigilant about checking where their message is going, but it appears that all major mail clients support our application of the reply-to field.
I also realize this is not technically a bug report forum, but it seemed like the best chance to indicate an issue considering I could not find a direct feedback link for the Mail app. Thanks!