Aggressive Server Spam Filter, New Default Spam Mailbox not Getting Filtered?

I have 5 accounts, iCloud, 3 Gmail, and one work account for my own company. [Sequoia 15.2, Mail 16.0, SS 3.1] Recently noticed spam to my company domain account going to a new “Spam” mailbox in Apple Mail rather than the “Junk” mailbox recommended and used by other 4 accounts. I checked my setup and it appears OK, and the other 4 accounts are handled by SpamSieve normally.
Seems like a server side config issue (they have an absurdly unconfigurable spam filter) and I am about to find a new email host as a result. I tried turning on “Filter other mailboxes” in Spam Sieve, but seems to have no effect. What am I missing?

If the server spam filter is correctly moving spam messages to the Spam mailbox there’s nothing that you necessary need to change.

However, normally, I would suggest that you set the Junk mailbox for that account in Mail to the same mailbox that the server filter is using.

Filter spam messages in other mailboxes is intended for filtering messages in regular mailboxes, not Spam/Junk mailboxes.

Thanks, that did it. I don’t recall ever UNdoing that, or designating a Spam folder since the big change to Junk with the SS mail extension, but it’s working now. One oddly persistent (but empty) “spam” mailbox remains in my Favorites sidebar, but I can ignore it now.

Maybe it’s part of another account that also needs its Junk mailbox set? Or you could just remove it from the favorites.