Upgraded to Big Sur yesterday. Since immediately afterwards, spam is marked properly but not moved out of the inbox. It appears in the SpamSieve log, properly flagged, just doesn’t move to Spam. I did the option-install Apple Mail Plug-In trick but no change. Rules are active, and I can see the SpamSieve is evaluating correctly.
Any good ideas for next steps on this?
Which version of macOS were you using previously?
It sounds like SpamSieve is working fine but that you’re running into a Mail issue (which some people started seeing with Catalina) where Mail rules don’t move messages. It may help to set the rule to use a Spam mailbox on the mail server rather than under On My Mac. There’s more troubleshooting information here.
I seem to have the “local spam mailbox” problem. I created multiple rules to send to a spam mailbox on each server and that seems to be working correctly. Inconvenient, and increased maintenance, but working. Thanks for your help.
Came here in search of the same issue after my upgrade to Big Sur. My original setup (with the problem) was:
After the upgrade to Big Sur, spam was staying in my Mail.app mailbox, but it took me a while to realize it because looking at my inbox through Spark or iOS Mail looked correct. Using the Gmail web interface also looked correct, so it’s simply a matter of Big Sur’s Mail.app doing something wrong.
I was able to keep my same setup, but using Michael’s hidden SpamSieve preference: x-spamsieve://default?k=AppleMailPlugInSetIsJunk&v=NO seems to have fixed the issue for me (for now).
I guess we’ll see what happens when 11.1 gets released next week 
Thanks all!
brad
Yes, I’ve heard this from some other customers on Big Sur, too. Mail on the Mac shows the messages in a different mailbox than the server and other devices.
Great! That seems much less necessary on Big Sur, which is why SpamSieve no longer turns it off by default, but in some cases it does still help.