Whoever solves this deserves a medal. This problem appears only in Smart Mailboxes that include a date criterion, such as "received today", or "received in the last month." What happens is that some messages with attachments are preceded (in the message list) by a second message containing a text version of the attachment. This mystery message is always listed as being from (No Sender), and the location is always listed as "Files". I have not been able to discern any other pattern, such as type of attachment, etc. If I switch the view to, say, my InBox, the (No Sender) message is not visible; everything is normal in regular mailboxes. I have tried deleting smart mailboxes and creating new ones, but the problem persists. This is an Exchange account, if that even matters. I've tried rebuilding mailboxes, recreating my Exchange account, reimporting my messages, etc. The other thing is that this is happening on two different computers: a MacBook Air running OS X 10.11.4 (El Capitan), and a Mac Pro (circa 2009) running Yosemite. I don't see it in iOS, but that version of Mail doesn't support smart mailboxes (except VIPS, I suppose, and I haven't seen the issue there).
Does anyone have any idea what might be causing this, and how to get rid of it?
MacBook Air (13-inch Mid 2013), OS X El Capitan (10.11.4)
