Computing | Computer Security | Linux Security | Industrial Control Systems Security | Penetration Testing | Email Exploit | internet-exploitation

telnet mail.test.server 25
220
HELO CENTAUR
250
MAIL FROM: <>
250
RCPT TO: <>
250
DATA

(press enter twice)

SUBJECT:

(Write body of email)

(hit enter twice)

.
250: email queued

The plain idea

This little transcript shows something that surprises most people: email is just text sent over a plain conversation, and you can type that conversation by hand. Using telnet to connect to a mail server on port 25 and typing the SMTP commands (HELO, MAIL FROM, RCPT TO, DATA) yourself, you can hand-deliver an email the same way a mail server does. It is the clearest possible demonstration of how SMTP (the Simple Mail Transfer Protocol) actually works underneath all the apps.

Why it matters

Doing this once demystifies email forever. You see that MAIL FROM and the “From:” header are different things, which is exactly why email is so easy to spoof (hence this page’s place in the security notes: an attacker can put anything in those fields). You also see why spam, phishing and sender-forgery were possible for decades, and why the modern anti-spoofing defences had to be invented.

An everyday analogy

It is like discovering you can walk up to the post office sorting counter and read out a letter for the clerk to write down and send. Handy to understand, but also alarming: nothing stops you claiming the letter is “from the Queen”, which is precisely the problem email spent years fixing.

A 2026 update

This hand-typed trick is now mostly a teaching exercise, not a practical attack, because the email world locked down:

• Port 25 is widely blocked for ordinary users; real sending uses authenticated submission on port 587 or 465 with a login.
• TLS encryption is expected, so most servers will not accept a plaintext telnet session (you would use openssl s_client -connect host:587 -starttls smtp to see the modern equivalent).
• SPF, DKIM and DMARC now let receiving servers check that a message really came from where it claims, so naive MAIL FROM spoofing lands in the spam folder or is rejected outright.

The protocol underneath is still exactly the SMTP conversation shown here; it just wears authentication and encryption now.

How it connects

• Internet Exploitation: email spoofing as a social-engineering tool.
• Encryption: the TLS and signing that secure mail today.
• Networking: SMTP as an application-layer protocol.

The truth revealed: email is, at bottom, a short polite text conversation between servers, and once you have typed it by hand you understand both how it works and why trusting “who it’s from” was never safe.