If you have upgraded to macOS Monterey 12.3, your MsgFiler mail plugins may have been disabled. If so, please upgrade to the latest versions of the plugins:
- MsgFiler Engine: Version 1.6.2 works with macOS 12.3. This is the plugin you should install if you are purchased MsgFiler from the Mac App Store.
- MsgFiler Classic: Version 2.6.2 works with macOS 12.3.
If you are running a previous version of macOS, such as Big Sur, Catalina or Mojave, you should not need to do anything.
MsgFiler 
Thanks, Adam, for being SUPER-responsive when I whined about the plugin being disabled with macOS 12.3, muchly appreciated!