SMTP is used widely to send and receive emails over small, as well as large, infrastructures (can be public or private). The protocol uses the Sender-SMTP process to send e-mails and the Receiver-SMTP process to receive emails. This makes SMTP a client-server-based protocol that runs over port 25. However, many mail server admins follow the secure practice of changing the default port number for SMTP to any other random port that prevents the server from sending any spams out there in the wild and even keep the server out-of-reach from malicious users.

Most commonly, an SMTP channel for mail transfer is created using a TCP three-way handshake that happens between two hosts, which is followed by a series of SMTP packets. For illustration purpose, I configured one SMTP server on 192.168.1.105 and a client on 192.168.1.104. The client will request the server to send an e-mail to an address known to the client. The server will respond to this request with numerical...