Based on what I’m reading, it would seem that Apple has deprecated the previous plugin framework and indicated that devs need to switch to their new MailKit framework. Per MacRumors:
MailSuite and SpamSieve and DEVONthink’s plugins are operating today in Mail 15. (Although MailSuite is a buggy and crash-prone as ever, but I suspect that is not Apple’s fault.)
I am guessing here but would think that if it shows up in Settings under the Extensions tab it’s using the new framework and the old framework if under General-Manage Plig-ins…
Yeah, I’m using Mail on Monterey and I have SpamSieve at least running. I’m considering nabbing MailSuite, but it’s that “will stop functioning in an unspecified future macOS release” that’s concerning me. I don’t want to buy something, get used to it, and have it vanish.
Y’know, I never thought of that. That would make a ton of sense. For me, SpamSieve is showing up under the latter - so logically not an extension.
Last time I checked the MailKit framework does not even remotely have the capabilities needed for those types of plugins. If Apple removed support for the legacy Mail.app plugin framework today, there is no way the developers could port them.
I wonder if Apple will be adding it later, or if this is a signal that people who want that different functionality should be looking for a different mail client?
Who knows, but let’s look at what is being done with Apple Reminders: it has become not only a capable application but also a framework on top of which many developers can use into their apps (not only GoodTask or Fantastical, see Memento or Butleroy)
Technical stuff aside, I don’t see any reason why Apple would not choose to do the same with MailKit but in this case create an ecosystem of apps that work within Mail.app, it’s just that we are in the middle of a transition.
I used MailTags and Mail Act On several years ago. Seems like Apple broke everyone’s plugins every time they updated Mail.app. And each time some weeks later there would be an update and they would start working again. But eventually I gave up and quit using any plugins.
For me, SpamSieve is showing up under the latter - so logically not an extension.