Email Explained from First Principles” – a very long reference that touches on anything you ever wanted to know about the email system (97,000 words – an estimated 8 hour read).

Notes section that will grow in the future:

  • The email Address Syntax section talks about the fact that the “local part” of an email address (to the left of the @ sign) can be just about anything. It can be a quoted string, so this would be valid: "@"@ef1p.com . Also, you can use an IP address for the domain portion so this would be valid: user@[192.0.2.123] . There are some notes about pitfalls and webmail providers who support/block this.
  • The “To” and “CC” fields you are seeing in a message can be faked just like the “from” address. This can create the illusion that the sender has a connection to people mentioned in those fields, without ever having to actually deliver the same message to them.