Simple Mail Transfer Protocol

• Email sending protocol
• SMTP commands
• SMTP Enumeration

SMTP is a TCP/IP mail delivery protocol. It transfers email across the Internet and across local networks. It runs on the connection-oriented service provided by TCP and uses the well-known port number 25. Below table lists some commands used by SMTP and their respective syntaxes.

SMTP uses mail exchange (MX) servers to direct mail via DNS. It runs on TCP port 25, 2525, or 587.

!Mail Transfer

Process

1. Your email app talks to your SMTP server.
2. It sends who it’s from, who it’s to, the message, and any files.
3. The server checks if it’s the same domain.
4. If not, it connects to the recipient’s SMTP server. If that fails, it waits.
5. The recipient’s server checks the message and passes it to their inbox server (POP/IMAP).
6. The recipient’s app pulls the email into their inbox.

Reference

1. https://blog.bityard.net/articles/2023/January/various-ways-of-sending-mail-via-smtp

Pentesting

SMTP (Simple Mail Transfer Protocol) Pentesting | Hackviser