SMTP
SMTP, or Simple Mail Transfer Protocol, is a protocol used for sending email messages between servers. It's a part of the Internet Protocol Suite and was initially defined in 1982 by Jon Postel in RFC 821. Here's a detailed look at SMTP:
History
- SMTP was first published as RFC 821 in August 1982 by Jon Postel, aiming to provide a reliable and efficient method for transferring electronic mail over the Internet.
- In 1995, RFC 1869 updated the protocol, introducing SMTP Service Extensions, which allowed for additional commands, authentication, and encryption.
- Further updates were made in 2008 with RFC 5321, which is the current standard, consolidating previous standards and clarifying many aspects of the protocol.
Functionality
- Mail Submission: SMTP is primarily used to send email from a mail client to an Email Server or between Email Servers. It uses a simple command/response interaction over a TCP connection.
- Commands: SMTP uses commands like HELO, MAIL FROM, RCPT TO, DATA, and QUIT to facilitate the sending of emails.
- Authentication: While originally designed without security in mind, extensions like SMTP-AUTH provide authentication capabilities to prevent unauthorized use of mail servers.
- Encryption: SMTP can operate over TLS (Transport Layer Security) for encryption, known as SMTPS or ESMTP with TLS.
How It Works
SMTP operates in three stages:
- Connection Establishment: The client connects to the server on port 25 (or 587 for submission) and initiates the session with a HELO or EHLO command.
- Mail Transaction: The client specifies the sender's address (MAIL FROM), the recipient's address (RCPT TO), and then sends the email content (DATA).
- Connection Termination: The transaction is completed with a QUIT command, closing the connection.
Extensions and Enhancements
- ESMTP: Extended SMTP (ESMTP) introduces extensions that provide additional functionality like authentication, pipelining, and support for larger message sizes.
- STARTTLS: This extension allows an SMTP server to use TLS to encrypt the session after the initial unencrypted connection is established.
- SMTP Authentication: Allows the sender to authenticate to the SMTP server to send emails, using mechanisms like PLAIN, LOGIN, or CRAM-MD5.
Security Concerns
- Due to its initial lack of security, SMTP has been prone to abuse for spamming. Over time, security measures like authentication and encryption have been added.
- SMTP servers can be configured to reject emails from unknown or untrusted sources as part of Anti-Spam Techniques.
Sources
Related Topics