In Evan's talk, he showed how the mailing list software he used could report how many recipients had read each message. I asked how that worked. This devolved into a discussion with more heat than light. So I looked into this a bit. I was puzzled because, generally speaking, mail servers talk to each other but Mail User Agents generally don't. And yet only MUAs know when a message is read. RFC 8098 <https://tools.ietf.org/html/rfc8098> describes a MIME mechanism for "Message Disposition Notification". For example, an email sender can request the the user's MUA send back notification when a message is read. Apparently some MUAs implement this and some don't. Alpine, my MUA, can issue an MDN request but will not respond to one. Mozilla products seem to be able to optionally respond. I suspect that Outlook unconditionally responds. There are other variations on this <https://en.wikipedia.org/wiki/Email_tracking> I'm going to try to set DSN on this message.