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Email Protocols

What are Email Protocols?

An email protocol is a group of rules and regulations that ensure that email messages are properly created, transmitted, and received between different servers, machines, and email clients. Email protocols create standards that messages should follow to reach the recipients in their original state. The following are the three standard email protocols:

SMTP. Simple Mail Transfer Protocol (SMTP) establishes how the message gets from the sender to the email server.1 Also, it is used by a mail transfer agent (MTA) to deliver emails between servers. This process is called mail relaying (or SMTP relay). SMTP is only meant for sending email messages. SMTP workflow consists of commands sent by the SMTP client and corresponding replies by the SMTP server. SMTP conversations consist of three stages:

  1. SMTP handshake. The SMTP client establishes a TCP connection with the SMTP server. Once the server replies with 250, the handshaking starts. This stage ends when the server confirms the recipient’s address.
  2. Email transfer. Code 354, as a response to the DATA command, launches the transfer of the email. Once the server gets a final dot, the message is transferred.
  3. Termination. Client and server say goodbye to each other using the command QUIT and code 221, respectively.

POP3. Post Office Protocol 3 (POP3) is a simpler way to access mailboxes. POP3 is the third version of POP and has a low dependency on Internet connections. POP3 protocol downloads emails from the server to the local computer, so you can read them even offline. The server deletes messages once they are retrieved. POP3 connection progresses as follows:

  1. The client connects to the server (AUTHORIZATION State)
  2. The client retrieves new emails (TRANSACTION State)
  3. The server deletes the stored messages (UPDATE State)
  4. The client disconnects from the server

IMAP. Internet Message Access Protocol (IMAP) is a standard protocol for accessing emails and is a client/server protocol. With IMAP, emails are received and held by the Internet server. IMAP is suitable for different devices simultaneously and requires users to be constantly connected to the Internet. When a user accesses the mailbox, the user is actually connected to an external server. This is more advantageous when there are multiple users. IMAP can work over a relatively low internet connection since it only downloads email messages from the server when a user has requested to read a specific email.

The IMAP client/server interaction progresses as follows:

  1. A recipient’s mail client connects to the server the message is stored on
  2. The recipient can see the message headers of all the emails on the server
  3. If the recipient chooses a particular message to read, IMAP downloads it on demand

1 Tsvetukhin, 2023, “Difference Between IMAP, POP3, and SMTP Email Protocols”