I have set up the GPGTools on OS X 10.11 and they work perfectly fine in Apple Mail, as I have verified by testing with two of my email addresses. Even in Outlook 2016 with the gpg4win plugin on my Windows 10 machine the emails are decrypted automatically and signatures are verified correctly.
It doesn't work the other way round though: Emails encrypted in Outlook 2016 using the gpg4win plugin do NOT get decrypted (automatically) at the receiving email address. Not in Outlook, and not in Apple mail either. Instead, I'm shown the encrypted message between the PGP message lines.
BTW: After creating the key pair with the GPGTools in OS X, I exported the two pairs for my two email addresses and imported them in Kleopatra (gpg4win) on my Windows 10 machine.
Can anybody, please, tell me what's going wrong? Why is decryption working correctly when emails are sent from Apple Mail (GPGTools, OS X), but not when sent from Outlook 2016 (gpg4win, Windows 10)? Any help is appreciated!