knowledge/technology/internet/SMTP.md

aliases	obj	wiki	rfc
Simple Mail Transfer Protocol	concept	https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol	https://datatracker.ietf.org/doc/html/rfc5321

Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is a protocol used for the transmission of electronic mail (email) over a computer network. SMTP is a text-based protocol that facilitates the sending and receiving of emails between servers and is a fundamental component of email communication on the internet.

Key Concepts

1. Message Transfer:

• SMTP is primarily responsible for the transfer of email messages between a sender's email client (Mail User Agent - MUA) and a recipient's email server (Mail Submission Agent - MSA) and between email servers (Mail Transfer Agent - MTA).

2. Client-Server Interaction:

• SMTP operates on a client-server model, where an email client sends messages to an email server for delivery. Servers communicate with each other to relay messages.

3. Port Number:

• SMTP uses port 25 for unencrypted communication. Port 587 is commonly used for secure communication with Transport Layer Security (TLS).

4. Commands and Responses:

• SMTP communication involves a series of commands and responses between the client and server. Common commands include HELO (client identification), MAIL FROM (specify sender address), RCPT TO (specify recipient address), DATA (begin message data input), and QUIT (terminate the session).

5. Relaying:

• SMTP allows for the relaying of messages between different email servers, facilitating the delivery of emails across different domains.

SMTP Operation

1. Handshake:

• The SMTP session begins with a handshake, where the client identifies itself to the server using the HELO or EHLO (extended hello) command.

2. Sender and Recipient Specification:

• The client specifies the sender's address using the MAIL FROM command and the recipient's address using the RCPT TO command.

3. Message Transmission:

• The actual message is transmitted using the DATA command, allowing the client to send the message body and headers.

4. Session Termination:

• The session concludes with the QUIT command, indicating the end of the SMTP communication.