I've uninstalled and reinstalled every version of Mailhub available, and I have even wiped out the OS and reinstalled it from scratch. Mailhub was the very first app I tried to install but it either fails to install or the "Manage Plugins" button never shows up in Mail. Either way, I am not able to use it.
I was having the same no "manage plugins" button problem with Mojave – installing Small Cubes Mail Suite Plug Ins brought the button back for me and let me install MailHub
I am having the same problem . No "manage plugins" button problem with Mojave