SMTP
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Simple Mail Transfer Protocol operates in the Application Layer and uses TCP ports 25, and 465/587 for encrypted communication. It is the primary protocol used for sending and relaying email messages across the Internet, by transferring messages between mail servers and sending client applications. SMTP does not handle email retrieval, so it works alongside IMAP or POP3 for receiving emails.
SMTP follows a client-server model in the following way:
SMTP Client: The system, such as an email application or mail transfer agent, sends emails to an SMTP server
SMTP Server: The mail server processes, routes, and delivers email messages to the recipient’s mail server
To facilitate email transfer, SMTP uses a series of commands and responses between mail servers. Some of them are:
HELO/EHLO: Identifies the sending mail server to the recipient server
MAIL FROM: Specifies the sender’s email address
STARTTLS: Switch plaintext connection to encrypted connection
RCPT TO: Defines the recipient’s email address
DATA: Transfers the content of the email
RSET: Abort initiated transmission but keep the connection
VRFY: Check if a mailbox is available for message transfer
QUIT: Ends the session
SMTP servers also use authentication and encryption mechanisms, such as SSL or TLS to secure email communication.
Connect to an SMTP server using the protocol
Check for existing users
Set sender and receiver
Send data
Close connection