"Manage plug-ins" button gone from Mail in Mojave

Hi all,

Since upgrading to Mojave, I can't seem to activate a Mail plug-in because there is no longer any "Manage plug-ins" button on my system for some reason. All the Mail plug-in developers say you need to activate plug-ins manually now on Mojave and that you need to click on this button in the Preferences > General pane, but I don't have one anymore (see screen grab below). I remember having that button before upgrading to Mojave.

Any ideas?

Cheers,

Grey