Is there any particular reason that “junk” can’t be used as the default folder for SpamSieve rather than a mailbox in “On My Mac”. The application didn’t seem to offer it because it is entered as text of the mailbox name. I modified the rule to Move Message to Junk. Is there any reason that wouldn’t work?
The reason it’s not the default is that Mail will only show the Junk mailbox if its own junk mail filter is active. When using SpamSieve, you should not use Mail’s filter because it will make it hard for you to correct SpamSieve’s mistakes. Also, you need to remember not to use the Junk and Not Junk toolbar buttons, since those work with Mail’s filter. Thus, for most people it’s much simpler to turn Mail’s filter off when using SpamSieve.
That said, you can have SpamSieve use the Junk mailbox if you disable Mail’s junk filter.