Short Note: Simple Mail Transfer Protocol (SMTP)

SMTP stands for Simple Mail Transfer Protocol. It is an application-layer protocol used to send email from a mail client to an outgoing mail server and to relay messages between mail servers.

Working

1. The sender composes a message in a mail application.
2. The client submits it to an outgoing mail server, commonly using authenticated submission on port 587 with STARTTLS or implicit TLS on port 465.
3. The sending server looks up the recipient domain's MX record in DNS.
4. It relays the message to the recipient's mail server, commonly using port 25 between servers.
5. The recipient retrieves the message using IMAP or POP; SMTP is primarily for sending and relaying, not mailbox retrieval.

Basic commands include EHLO/HELO, MAIL FROM, RCPT TO, DATA and QUIT.

Advantages: standardized mail transfer, store-and-forward delivery, and support for relaying between different providers.

Limitations: SMTP alone does not guarantee confidentiality or prevent spoofing. TLS, authentication, SPF, DKIM and DMARC help secure transport and improve domain-level anti-spoofing.

Conclusion: SMTP is the standard protocol for sending and relaying email, while IMAP and POP are commonly used to access received messages.