I’m assuming they are spam as it doesn’t seem like I’m losing any legitimate messages. I’ve contacted my host provider (dreamhost) and they’ve restored my mailbox many times because sometimes it results in existing messages in my inbox to dissipear. That doesn’t happen as often but I do get the (No Sender) (No Subject) messages almost every day.
Is this possibly a SpamSieve bug? I’m using 2.9.7 and 10.8.3