I’m one of the unlucky Sonoma users whose SpamSieve mail extension doesn’t work out of the box. Fortunately, it works ok after ticking “filter in other mailboxes.” I never have more than a couple of dozen messages in my inbox and don’t really get a lot of spam anymore. But I’ve been a paid SpanSieve user since 2004. The cost of the SpamSieve 3 update seems like a small price to pay for nearly 20 years of spam-free email. Thanks!
SpamSieve 3.0.1b2 can now automatically filter new messages in the inbox that Mail failed to send to the Mail extension. So, in most cases, you no longer need to do anything to work around this Mail bug.