> The problem seems to occur with all sorts of content types and encoding. > I've now installed Office 2016, and the same problem occurs, only worse. Irrelevant. It's Exchange that messes things up. Switching to tb will most likely not help. Set yourself up with a personal email account that is not going through Exchange, subscribe to this list, and then compare the full messages, headers and all, from both sources. This place of ork uses O365, and the Exchange server is adding so much crap to headers and body that message size triples or worse. Certain MIME types break. MS support doesn't have the technical background to understand these issues when you open a case.