Simple Mail Transfer Protocol (SMTP) is an Internet standard for email transmission. It is a protocol used to send and receive email messages between mail servers and is widely used for sending emails over the Internet. SMTP defines the rules and procedures for how email is transferred and delivered.
Overall, SMTP is a fundamental protocol for email communication, allowing email messages to be sent, routed, and delivered across different mail servers. It provides a reliable and standardized method for exchanging emails over the Internet.
The description of the various aspects of SMTP
Basic Operation: SMTP operates on the client-server model, where an SMTP client initiates a connection with an SMTP server to send an email. The client establishes a TCP connection on port 25 (by default) with the server and begins the email transaction.
SMTP Commands: SMTP communication consists of a series of commands and responses between the client and server.
Some of the important SMTP commands and functions
HELO (or EHLO): The client introduces itself to the server and provides its domain name.
MAIL FROM: The client specifies the sender’s email address.
RCPT TO: The client specifies the recipient’s email address.
DATA: The client sends the actual email content, including headers and body.
QUIT: The client terminates the session.
Email Routing: SMTP handles the routing of email messages between different mail servers. When an SMTP server receives an email, it checks the domain of the recipient’s email address and determines the next server to forward the email to. This process continues until the email reaches the server responsible for delivering it to the recipient’s mailbox.
Relaying and Authentication: SMTP servers employ various techniques to prevent unauthorized relaying of email messages. Open relays, which allow anyone to send emails through the server, are generally disabled to prevent spam abuse. SMTP servers may also require authentication from clients before allowing them to send emails.
MIME and Attachment Support: SMTP supports Multipurpose Internet Mail Extensions (MIME), which allows emails to carry various types of content beyond plain text, such as attachments, images, and multimedia. MIME headers are added to the email to indicate the content type and encoding.
Security: SMTP originally lacked built-in encryption, making it vulnerable to eavesdropping and tampering. However, modern SMTP implementations support Transport Layer Security (TLS) or Secure Sockets Layer (SSL) encryption, commonly referred to as SMTPS, to secure the communication between the client and server.
Email Delivery Status Notifications (DSNs): SMTP supports Delivery Status Notifications (DSNs), which provide information about the delivery status of an email. DSNs allow senders to be notified if an email was successfully delivered, delayed, or failed to be delivered.
Response Codes
After each command, the SMTP server sends a response code to indicate the success or failure of the command. The response codes are three-digit numbers, where the first digit indicates the response category:
2xx: Successful completion of the command.
3xx: Intermediate response, additional input is expected.
4xx: Temporary failure, the command cannot be processed at the moment.
5xx: Permanent failure, the command cannot be processed at all.