Simple Mail Transfer Protocol (SMTP)

SMTP is used widely to send and receive emails over a small network. The protocol uses the Sender-SMTP process to send emails and the Receiver-SMTP process to receive emails. This makes SMTP a client-server-based protocol that runs over port 25.

Typically, an SMTP channel for mail transfer is created through a successful TCP three-way handshake followed by a series of SMTP packets:

In our lab, we have an SMTP server configured at IP 192.168.1.105 and a client at IP 192.168.1.104. The client will request the server to sends an email to an address known to the client. The server will respond to this request with numerical code, followed by a brief response parameter.

Dissecting SMTP communication packets

Using the Netcat client from a Kali Linux machine, I will connect to the SMTP mail service running on a Windows machine. After a successful three-way handshake, the server will respond with numerical codes with a short summary. Follow these steps to the send...