“Incompatible Plug-ins Disabled” with Apple Mail on Monterey

Also having trouble with Spam Sieve and Monterey. Mail keeps disabling Spam Sieve extension, saying it’s incompatible. Using v2.9.49

I have uninstalled, and manually deleted from user/library/mail/bundles/ and /library/mail/bundles, yet Mail still says the plug-in is installed.

Probably you need to reset Mail’s private plug-ins folder.