I have discovered a wonderful resource about email; perhaps the best deep discussion of email I have ever read!
From the concepts section of the article:
Before diving into the technical aspects of email, let’s first look at email from the perspective of its users.
The purpose of email is to send messages over the Internet. A message is a recorded piece of information which is delivered asynchronously from a sender to one or several recipients. Asynchronous communication means that a message can be consumed at an arbitrary point after it has been produced, rather than having to interact with the sender concurrently. A message can be transmitted with a physical object, such as a letter, or with a physical signal, such as an acoustic or electromagnetic wave. While humans have delivered messages in the form of objects for millennia with couriers and pigeons, it’s only since the invention of the optical telegraph in the late 18th century and the invention of the electrical telegraph in the middle of the 19th century that we can signal arbitrary messages over long distances. The fundamental principle of communication stayed the same over all those years: You can either start a new conversation or continue an existing one by replying to a previous message.
A mailbox is a box for incoming mail (also called an inbox), into which everyone can deposit messages but ideally only the intended recipient can retrieve them. In some countries, the privacy of such messages is legally protected by the secrecy of correspondence.
There are three things that set email apart from the traditional postal system, which is sometimes also referred to as snail mail:
Email conveys digital data, whereas a letter is a physical item. The former is much more useful for further processing. Email enables instant global delivery at a marginal cost of zero. The only fee you pay is for your access to the Internet. Mailboxes for email are provided and operated by companies, which are called mailbox providers. While you could operate your own server since email is an open and decentralized system, this is rarely done in practice for reasons we discuss later on. Terminology: Earlier versions of this article used the term email service provider (ESP) instead of mailbox provider. Since the former term is also used to refer to email delivery vendors, I decided to replace it with the latter term. Somewhat confusingly, mail service provider (MSP) is a synonym for mailbox provider even though mail and email are used interchangeably in the context of email.