SMTP (Simple Mail Transfer Protocol)

SMTP (Simple Mail Transfer Protocol) is the standard protocol used on the Internet for the transmission of emails between systems. This protocol is responsible for sending messages from one mail server to another; mail servers and mail transfer agents use SMTP to send and receive email messages on behalf of users. SMTP only handles the sending of messages, not their reception or storage.

Key features of SMTP:

• Sending email: SMTP is used exclusively for sending messages. For receiving email, protocols such as POP3 or IMAP are used.
• Network port operation: Normally, SMTP operates over port 25 for unencrypted connections and ports 587 or 465 for SSL/TLS encrypted connections.
• Simple and efficient: SMTP is a relatively simple protocol, designed to transfer messages quickly and efficiently.

How SMTP works:

1. Connection and communication: An email client (or server) connects to an SMTP server and sends a message along with routing instructions, such as the recipient’s email address.
2. Routing: The sending SMTP server determines where the message should be sent based on the recipient’s domain and queries DNS to find the appropriate receiving mail server.
3. Message transfer: The SMTP server transfers the message to the receiving SMTP server (or an intermediate server). This process may involve several hops if intermediate servers are needed.
4. Delivery to the recipient: Once the email reaches the recipient’s SMTP server, the SMTP process concludes. The receiving server then uses POP3 or IMAP to store the message until the recipient retrieves it.

SMTP security:

• Authentication: SMTP supports extensions for authentication that prevent abuse of the mail system, such as sending spam or unsolicited email.
• Encryption: The use of TLS (Transport Layer Security) is common to encrypt the SMTP connection and protect message data during transmission.
• SPF and DKIM: These are additional technologies used to validate email messages and ensure they have not been altered in transit, and to verify that the sending server is authorized by the sender’s domain.

SMTP remains a fundamental pillar of email infrastructure, and despite its age, it has adapted to modern network security and efficiency requirements.