The GPG suite which includes the Apple Mail plugin is not yet compatible with Sierra. It will I am sure get an update eventually to resolve this but you will have to wait for it. See https://gpgtools.org/
Oct 23, 2018 - GPGMail 3.0.1 (macOS High Sierra & macOS Mojave). Mail could crash when trying to verify a PGP signed message [#1003]; Fixes a crash in.
![Pgp For Mac Sierra Pgp For Mac Sierra](/uploads/1/2/5/6/125684978/342976078.png)
This is why I personally prefer using S/MIME rather than PGP or GPG. S/MIME is built-in to Apple Mail so other than getting a S/MIME email certificate nothing else is required. S/MIME is in fact built-in to Outlook and the iPhone as well.
I don't know why people waste time on GPG/PGP when it is so much easier to do S/MIME. I guess it is inertia since PGP pre-dates S/MIME.
Oct 14, 2016 2:21 AM