Ask HN: Books on how Email works and how to program services around it

Can you please recommend me Books/Ebooks on the Email and things around it?

Principles of how it works, how servers connect to each other, security, extensions, implementation details, programming services around Email.

I diligently searched on the web and google, but only found Programming Internet Email which seems to be quite old.

There are tons of books on topics like JavaScript, Databases, Python, and so on, but no books on email?

Also, I'd like to hear opinions if you think that can email be sexy and viable as a basement for a startup these days? I find email the only tool that have been with me for years and I am sure will be with me for years ahead.

Please advise and share your opinions.