I occasionally use DeltaChat to communicate with someone who doesn’t have it but has email. It is actually one of its declared functions. The communication looks like with other chat apps from my point of view. So I see the conversation in similar bubbles like those other chat apps. But the partner on the other side only sees the lot of individual e-mails.
And since it’s still one conversation for me, I don’t write greetings and goodbyes every time. And for someone who only sees e-mails, it looks like rudeness. Therefore, I suggest that in the settings of each participant there could be a header as salutation and a footer as farewell. These would be automatically inserted into e-mails so that the communication does not appear impolite to the other partner, but they would not be visible in the conversation screen. And since the way of addressing and saying goodbye specifically applies to different participants differently each time, it would be necessary to set them not in general, but for each partner separately.