How to Send a Test Email Using Telnet and EHLO

Introduction

When something goes wrong with email delivery — messages bounce, get delayed, or never arrive — you often need to test whether your mail server can actually send emails. One of the most reliable and low-level ways to do this is by manually speaking the SMTP language using Telnet.

The EHLO command (Extended Hello) is the modern way email clients/servers introduce themselves. It tells the receiving server what features (like authentication, size limits, or TLS) the sender supports. This guide walks you through sending a complete test email using only Telnet — no extra tools needed.

This method is perfect for quick troubleshooting, learning SMTP, verifying firewall rules, testing new mail servers, or proving delivery problems to hosting providers.

Why Use Telnet + EHLO for Testing?

• Works on any Windows, Linux, or macOS machine with Telnet client
• No scripts, no software installation required
• Shows exactly what the server replies at every step
• Helps diagnose authentication, relay, SPF/DKIM, or blacklist issues
• Teaches you how real email servers communicate under the hood

Telnet sends everything in plain text — never use it with real passwords over untrusted networks. For production testing, prefer tools like OpenSSL or swaks.

Prerequisites

• Windows computer (or any OS with Telnet client enabled)
• Access to an SMTP server (your own mail server or a relay like smtp.gmail.com, smtp.office365.com, etc.)
• Port 25 (or 587 for submission) open outbound from your machine
• Know your SMTP server hostname/IP and port
• A sender email address you control (and ideally one recipient address)

On Windows 10/11, enable Telnet Client: Settings → Apps → Optional features → Add a feature → Telnet Client.

Full Step-by-Step: Sending a Test Email via Telnet + EHLO

We will connect to a public test SMTP server (or your own) and send a simple message.

Step 1: Open Command Prompt

Press Windows + R → type cmd → Enter

Step 2: Connect to the SMTP Server

telnet smtp.gmail.com 587

Common ports: 25 (relay), 587 (submission with STARTTLS), 465 (SSL). Use 587 for most modern servers.

Step 3: Greet the Server with EHLO

EHLO test.yourdomain.com

You should see a list of supported features like:

250-smtp.gmail.com at your service, [your IP]
250-SIZE 35882577
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8

Step 4: Start Encrypted Session (if server supports STARTTLS)

STARTTLS

Server replies: 220 2.0.0 Ready to start TLS

After STARTTLS, Telnet becomes encrypted — you won't see what you type anymore. This method stops here for plain Telnet. For encrypted testing, use OpenSSL instead (covered later).

Step 5: For Plain/Text Testing (Port 25, no encryption)

Use port 25 on servers that still allow plain text (many internal/corporate servers do):

telnet mail.yourcompany.local 25

EHLO workstation.yourcompany.local

Step 6: Tell Who Is Sending

MAIL FROM:<user@yourdomain.com>

Server should reply: 250 2.1.0 OK

Step 7: Tell Who Is Receiving

RCPT TO:<test@yourdomain.com>

Reply: 250 2.1.5 OK (or 550 if rejected)

Step 8: Start Composing the Message

DATA

Server replies: 354 Go ahead

Step 9: Type the Email Headers & Body

From: Test User <user@yourdomain.com>
To: Recipient <test@yourdomain.com>
Subject: Manual SMTP Test Email

Hello,

This is a test message sent manually using Telnet and EHLO.
Testing delivery from my workstation.

Best regards,
Your Name
.

Important: End with a single period . on its own line and press Enter.

Step 10: Finish the Session

QUIT

Server replies: 221 2.0.0 Bye

Common SMTP Reply Codes You Will See

Troubleshooting Common Problems

• Connection refused / timeout → Firewall blocking outbound port 25/587 or server down
• 554 Relay denied → Server requires authentication (use port 587 + AUTH LOGIN or use a relay that allows your IP)
• No response after EHLO → Try HELO instead (older servers)
• STARTTLS hangs → Telnet cannot continue TLS — switch to OpenSSL

Many public providers (Gmail, Office 365) require authentication and STARTTLS on port 587 — plain port 25 is usually blocked or restricted.

Conclusion

Using Telnet with the EHLO command gives you complete visibility into how SMTP really works. While modern tools like swaks, Thunderbird’s test feature, or PowerShell’s Send-MailMessage are more convenient for daily use, understanding the raw SMTP conversation helps you diagnose delivery problems faster and deeper.

Practice this method on your own internal mail server or a test relay first. Once comfortable, you'll be able to quickly verify whether the issue is with your server config, DNS (MX/SPF/DKIM), firewall, or the recipient's side.

Master this skill — it remains one of the most powerful low-level troubleshooting techniques in email administration.