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Simple Mail Transfer Protocol (SMTP)

SMTP stands for Simple Mail Transfer Protocol. It is a communication protocol used for sending and receiving email messages between email servers on the internet.

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SMTP is responsible for transferring outgoing mail from a sender’s email client to the appropriate email server, which then forwards the email to its intended recipient’s email server. The recipient’s email server then uses a different protocol, such as POP3 or IMAP, to retrieve the email and deliver it to the recipient’s email client.

SMTP uses a set of standard commands and responses to transfer email messages. These commands include MAIL, RCPT, DATA, and QUIT, among others. The protocol also includes mechanisms for handling errors and ensuring reliable delivery of email messages.

SMTP is an essential part of the email infrastructure and is used by millions of users and businesses around the world to send and receive email messages every day.

SMTP is a standard protocol that defines the rules for how email messages are sent between servers on the internet. It was first introduced in 1982 and has undergone several updates and revisions since then. Today, SMTP is widely used by email clients, email servers, and other email-related services to transfer email messages between computers over the internet.

SMTP is a text-based protocol that uses a series of commands and responses to transfer email messages. When a user sends an email message from their email client, such as Gmail or Outlook, the email client connects to their email server using SMTP. The email client then sends a series of SMTP commands to the email server to transfer the email message.
These commands include:

HELO: This command initiates a connection with the email server and identifies the sender’s email client.
MAIL FROM: This command specifies the email address of the sender.
RCPT TO: This command specifies the email address of the recipient.

DATA: This command begins the transfer of the email message data.
QUIT: This command terminates the SMTP connection.

Once the email server receives the email message, it forwards it to the appropriate email server responsible for delivering the message to the recipient. This process continues until the email message reaches its final destination.

SMTP also includes mechanisms for handling errors and ensuring reliable delivery of email messages. For example, if an email server cannot deliver a message to a recipient, it may send a bounce message back to the sender indicating that the message was undeliverable. SMTP also supports authentication mechanisms, such as SMTP-AUTH, which allow email servers to verify the identity of the sender and prevent unauthorized access to email accounts.

Overall, SMTP is an essential part of the email infrastructure and is used by millions of users and businesses around the world to send and receive email messages reliably and securely over the internet.

SMTP commands

The most common SMTP commands that are used to transfer email messages:

HELO: This command is used to initiate a connection between the sending and receiving email servers. The sending server identifies itself using this command.
MAIL FROM: This command is used to identify the email address of the sender of the email message.
RCPT TO: This command is used to identify the email address of the recipient of the email message.

DATA: This command is used to start the transmission of the email message itself.
SUBJECT: This command is used to specify the subject line of the email message.
CC: This command is used to specify additional recipients of the email message who will receive a copy of the email.

BCC: This command is used to specify additional recipients of the email message who will receive a blind copy of the email. The recipient list is not visible to the other recipients.
QUIT: This command is used to terminate the SMTP session.

SMTP also supports additional commands for authentication and encryption, such as AUTH and STARTTLS, respectively. These commands are used to provide additional security when sending email messages over the internet.

It’s worth noting that these SMTP commands are sent as plain text, so it’s important to use encryption and other security measures to protect the contents of email messages and prevent unauthorized access.

SMTP-AUTH

SMTP-AUTH (Simple Mail Transfer Protocol Authentication) is an extension of the SMTP protocol that allows email servers to authenticate users before allowing them to send email messages. SMTP-AUTH is used to prevent unauthorized access to email accounts and to ensure that email messages are sent only by authorized users.

SMTP-AUTH works by requiring users to provide a username and password before they can send email messages through an email server. When a user attempts to send an email message, the email client sends a login request to the email server using the SMTP-AUTH command. The email server then prompts the user for a username and password, and if the user provides valid credentials, the email server allows the message to be sent.

SMTP-AUTH can be used with a variety of authentication methods, such as MD5, CRAM-MD5, and PLAIN. These methods encrypt the user’s password before it is sent over the internet, ensuring that the password cannot be intercepted by unauthorized parties.

SMTP-AUTH is an important security feature for email servers, as it helps prevent unauthorized access to email accounts and helps ensure that email messages are sent only by authorized users. It is widely supported by modern email clients and email servers and is commonly used in conjunction with other security features, such as encryption and spam filtering, to provide a secure and reliable email service.

Further reading How secure is SMTP email?