Some macOS 10.13 users have reported that when they are away from a Mac that is either on or in Power Nap, Mail stops downloading new messages from the server, and thus SpamSieve stops cleaning out the inbox. As soon as they sit down at the Mac and unlock the screen, Mail starts processing the messages again. This seems to be caused by a macOS bug that can be worked around by going to System Settings ‣ Security & Privacy ‣ General and toggling the Require password checkbox off and then on again.