I have the same problem (Sonoma 14.0, Mail 16.0, SpamSieve 3.0), but I can not activate the “Select mailboxes to filter” workaround as I repeatedly get the following error message when the window opens after pressing the button.
The operation couldn’t be completed. (Foundation._GenericObjCError error 0.)
No mailboxes appear in the list, just the error message that opens up again after being closed.
I have used SpamSieve since 2014. I have 8 inboxes and the biggest one that receives the most spam has almost 82 000 messages in it. It is basically unfiltered before reaching Mail. The next biggest one is a Hotmail/Outlook inbox where there sometimes is a miss in their filtering.
I am now manually removing the incoming spam with “training”.
The next SpamSieve public beta will fix this error message so that we can see exactly what failed. But I think the most likely reason is that Mail is unable to handle the large number of messages when SpamSieve asks it how many there are. I think it will help if you can archive most of the 82K messages out of the inbox.
OK, I will considering archiving. But as the problem is not to big, maybe 5-10 spam messages a day, I could wait until “Apple fixes the bug”. If SpanSieve would start to work as normal then, even with a large mailbox.
Great! One of the changes here is that an error reading part of the mailbox list from Mail no longer prevents you from accessing other parts of the mailbox list. So there may be an account or mailbox that’s omitted due to an error, and that would be shown in the Log window and in the error log (Help ‣ Open Error Log).