Having been using the Apple Mail plug-in with no issues. Now moving to a new M1 MacBook Air (yes, 1 month before the M2 is out - I have impeccable timing) running Monterey 12.4. Have installed DT 3.8.3 on the new machine, and installed the Apple Mail plug-in (4.0.6) - which DT shows as installed - but I can’t find a button to activate it in Mail’s preferences (I think it should be on the General tab, but it’s not there).
Please excuse me if I’ve mis-read your message or if you’ve already done below, but just to check…did you see “Manage Plug-ins” button on bottom left of General panel of Preferences?