I have two email accounts, both IMAP accounts. I have two Spam folders, one under “On My Mac” and one under one of the two email accounts. One email account has no Spam folder. This means I have to check and empty two separate Spam folders. Can I get rid of one of them?
SpamSieve only uses the Spam mailbox “On My Mac.” The other one may be used by your server-side junk filter. You could try deleting it, but it might not let you or might bring it back.
You could create a smart mailbox that shows all the messages are there in any of the Spam mailboxes. Then there would only be one place to check and empty.