Email: Explained from first principles
I love such thorough expositions, although it isn’t quite as thorough as I would like. Two examples that caught my attention in the parts I read:1. It doesn’t mention the old convention for writing an email address with display name as  john.doe@example.net (John Doe)

instead of:  John Doe 

2. It states that subject prefixes like “Re:” have no technical relevance. That’s not entirely true, because email clients recognize existing prefixes when replying/forwarding, in order to not add a redundant one. There are several issues here:- Localized email software sometimes uses a different prefix than “Re”, based on the local language. This is an issue when having an email thread between email clients who don’t recognize each other’s local-language prefixes. (Arguably, it would be better for everyone to stick with “Re” regardless of  language.)- Some email clients have the convention of adding a count to the “Re”, e.g. “Re[2]:”, “Re[3]:”, and so on. When you write an email client, you may want to consider recognizing those.Due to the variety in subject prefixes, some email clients allow users to configure a regex.