OK ... this is really interesting. I'm on a TB MacBook Pro (2016) and have continued having problems at work. We thought it might be related to some funky configuration profiles and the network antivirus system because I wasn't having any problems at home/off campus. In my office, though, I use an external display -- and that's the *only* place I have consistent problems with mail and messages. Out of the office (home or even elsewhere on campus), my mac runs trouble-free.