GPG Suite 2018.1
This release of GPG Suite addresses a bug in GPGMail for High Sierra.
In some cases Mail would no longer display any OpenPGP signed or encrypted messages. In addition, new messages were no longer fetched from the mail server until Mail was restarted.
Disclaimer: GPG Suite 2018.1 includes a new beta of GPGMail 3.0 for macOS High Sierra. The beta version of GPGMail 3.0 will expire, once the stable version is available. As with every beta of GPGMail unexpected behavior and crashes may occur.