Email system protocols

1. RFC 822 email format
RFC 822 defines the format used for email messages. RFC 822 defines the content submitted and transmitted by SMTP, POP3, IMAP, and other Email transmission protocols. An email defined in RFC 822 consists of an envelope and content. An envelope includes information related to transmission and delivery.
The email content includes the title and body.

2. SMTP, Simple Mail Transfer Protocol, Simple Mail transmission protocol
It is a standard protocol for transmitting emails over the Internet. It is used to submit and send emails. It specifies the standard exchange format for transmitting emails between hosts and the transmission mechanism of mails at the link layer.
SMTP is usually used to transmit emails from the client to the server, and from one server to another.

3. POP3, Post Office Protocol, and Post Office Protocol. The current version is 3rd.
It is the first standard protocol for transmitting emails over the Internet and an offline protocol. It provides the information storage function to save emails received by users and download and retrieve them from the email server.
POP3 provides the client with a sender Trust (user name and password), so that you can standardize the access to email.

4. IMAP4, Internet Message Access Protocol, and Internet Message Access Protocol. It is currently version 4th.
When the email client software runs on a laptop (accessing the Internet and email over a slow telephone line), IMAP4 is more suitable than POP3. When using IMAP, You can selectively download emails, or even some of them. Therefore, IMAP is more complex than pop.

5. mime, multi-purpose Internet Mail Extension
The SMTP transmission mechanism on the internet is based on a 7-bit binary ASCII Code and is suitable for sending text emails. Audio, image, and Chinese emails encoded in binary format must undergo ASCII conversion (encoding) before they can be correctly transmitted over the Internet.
Mime enhances the ability of e-mail messages defined in RFC 822 to transmit binary data. Mime encoding technology is used to convert data from an 8-bit format to a 7-bit ASCII format. (21cn.com)