A little warning only. Just detected after sending out a newsletter to some clients on the Mac that formatted mail messages that have been edited from being sent out earlier will not have the content visible on iOS later versions (thought it was just iOS 27, but also in some slightly older versions too). My guess is that they have a new implementation of HTML (from 26.5-26.6.2). Unfortunately few people who know the Mac well use Apple Mail and thus there is very little support out there. But be warned of editing old mail messages to be sent out!
Just a note that it might not always be the case and one recipient who uses iOS 26.6.2 and Proton mail could read the message body just fine. Different servers handles formatted mail differently as it was … but the majority objected to something in the old formatting that maybe now is deemed as insecure … .