When opening Mail this morning, I was surprised o have the alert from SpamSieve saying that the plugin was disabled. Thus, I reinstalled the plugin according to SpamSieve instructions and when verifying in Mail, the plugin list is still empty
My configuration is macOS Monterey version 12.4 (21F79) Mail Version 16.0 (3696.100.31)
Thanks for your answer.
In fact I’m a kind of stupid guy: I was not looking for the plug-in with the Manage Plug-ins… button where the plugin is actually listed and checked and SpamSieve works fine.
Sorry