I might have a new MacBook, which might have an M1 processor. Having installed DT3.8 on this hypothetical machine, I then might have gone ahead and installed the add-ons, including - of course - the Mail plugin. Which now resides in ~/Library/Mail/Bundles as it should. However, when I start Mail, there is no Manage Plugins… button in Preferences > General. Restarting the Mac, removing the bundle from the path above, reinstalling the Mail add-on, restarting the Mac, and doing all that a few times, has made no difference - the button remains elusive. There are a few reports dotted around the web of such problems, but none seem to relate to macOS Monterey (12.1 btw.) and the solutions I have seen would write to a plist which appears to be obsolete in 12.1.