Older Messages Not Automatically Moving to Spam

I’ve installed SpamSieve for Apple’s Mail program, and it seems to be working fine generally. Incoming messages are being flagged and filtered appropriately, however messages that hit my inbox over the weekend while I was gone (which are still marked as unread) are not being filtered and moved. Shouldn’t SpamSieve automatically be doing this once I open Mail? I checked all settings, and everything seems like it should be working right. Any help or tips would be appreciated.

Yes, I would expect that to work. If the messages are marked as unread, Mail should apply the rules to them.

Is your SpamSieve rule at the top of the list? If you change the name of the rule from “SpamSieve” to “Move to Spam” then Mail should move all the messages to the Spam mailbox without consulting SpamSieve. If that does not occur, then that would tell us that the problem is unrelated to SpamSieve.