How to scan unread mail that was received before SpamSieve install?

I can’t figure out how to have SS scan email that Apple Mail already had in inboxes before I installed SS. It’s working great on email that has arrived since I installed SS, but I need it to scan the 4,000 unread emails from before the install. Help, please.

SpamSieve only auto-scans new messages received after it was installed, because we don’t want to surprise people by moving around their old mail that they thought was settled. To have it scan old messages, you can batch-select them in Mail and choose Filter messages from the SpamSieve menu bar icon or Dock menu.

If you’re going to filter a large batch of messages, it’s best to temporarily turn off the Auto-train as needed option.

Thank you very much!