(I looked around for a topic to which I could add this note as a reply, but didn’t find anything that seemed appropriate; so I’m posting this as its own topic.)
I updated to SpamSieve 3.0.1 final and I could swear that now Apple Mail is working correctly with the extension: that is, with “Enable spam filtering using Mail extension” and “Check inboxes for new messages not sent to Mail extension”, we are filtering so promptly upon opening Mail or receiving mail that I could swear Mail is telling SpamSieve about new mail, rather than SpamSieve having to poll periodically.
So either Mail is behaving correctly now, or else this setup is so fast that I can’t tell the difference.
Also, just FYI, I’m using the drone setup so that when I’m checking mail on my iPhone I can categorize a message to be trained as spam, and that’s working fine too.
That’s possible, as reinstalling the app or updating macOS seems to tickle the plug-in registry or something, which sometimes gets Mail to use the extension again (for a while). You can see in the Info ‣ Origin section of the log whether a given message was processed via the Mail extension.
I can’t interpret what the Origin entry for a given message says. For one particular recent message, for instance, I see three log entries: two of them say “Mail 16.0/3722.214.171.124.4 (SpamSieve extension 1.0b1)” and one of them says “Tidbits (IMAP) ‣ INBOX in Mail 16.0 (filter inboxes)”. I don’t know what to make of that.
By the way, the log window looks like it should be capable of sorting messages but it isn’t. It would be really nice if the default order, or at least an achievable order, were the reverse of what it is now (so that newest was first; right now newest is last and I can’t change it).