Oh, wow, haven't heard of Apple Mail plugins in a decade! I really thought Apple would have deprecated them by now.

Mail plugins are kind of a special case, they're REALLY "legacy", so they work by sharing process memory and being able to access way more than they should, like all your messages, possibly even passwords if the developer is clever enough.

Also notoriously difficult to build, since the Mail internal API breaks from time to time, so they're a rarity these days.

Also notoriously difficult to install.