Test Postfix Server with "telnet" Client

Email Tutorials - Herong's Tutorial Examples

∟Test Postfix Server with "telnet" Client

This section provides a tutorial example on how to test Postfix server with the 'telnet' client command to send a SMTP request to the server on port 25. Postfix will delivery emails to the '~/Maildir' sub-directory for each user.

One way to test the Postfix server is to use the "telnet" client command. All we need is the IP address of the server and the SMTP port number, which is defaulted to 25. Here is how I did it.

1. Run the following "telnet" session on my CentOS computer where Postfix server is running.

herong$ telnet localhost 25
  Trying ::1...
  Connected to localhost.
  Escape character is '^]'.
  220 mail.herongyang.com ESMTP Postfix
ehlo localhost
  250-mail.herongyang.com
  ...
  250 SMTPUTF8
mail from: herong
  250 2.1.0 Ok
rcpt to: herong
  250 2.1.5 Ok
data
  354 End data with <CR><LF>.<CR><LF>
Hello world!
.
  250 2.0.0 Ok: queued as 5EABD266002B
quit
  221 2.0.0 Bye
  Connection closed by foreign host.

2. Make sure that the email is delivered to herong's email directory. The output shows Postfix handles local SMTP requests correctly. And it delivers emails to local email addresses correctly. Note that this email file will be moved the "cur" sub-directory in few minutes.

herong$ cd /home/herong/Maildir/new/

herong$ ls
1605266421.Vfd02I203e0c2M847016.localhost.localdomain

herong$ more 1605266421.Vfd02I203e0c2M847016.localhost.localdomain
Return-Path: <herong@example.com>
X-Original-To: herong
Delivered-To: herong@example.com
Received: from localhost (localhost [IPv6:::1])
  by mail.herongyang.com (Postfix) with ESMTP id 5EABD266002B
  for <herong>; 15 Nov 2023 06:19:34 -0500 (EST)
Message-Id: <20231113111946.5EABD266002B@mail.herongyang.com>
Date: 15 Nov 2023 06:19:34 -0500 (EST)
From: herong@example.com

Hello world!

3. Open firewall to handle remote SMTP requests on port 25.

herong$ sudo firewall-cmd --zone=public --add-service=smtp
herong$ sudo firewall-cmd --runtime-to-permanent

4. Run the following "telnet" session on my macOS computer which is connected to the CentOS computer.

herong$ telnet 192.168.1.100 25
  Trying 192.168.1.100...
  Connected to 192.168.1.100
  Escape character is '^]'.
  220 mail.herongyang.com ESMTP Postfix
ehlo 192.168.1.11
  250-mail.herongyang.com
  ...
  250 SMTPUTF8
mail from: herong
  250 2.1.0 Ok
rcpt to: herong
  250 2.1.5 Ok
data
  354 End data with <CR><LF>.<CR><LF>
Hello again world!
.
  250 2.0.0 Ok: queued as D86E9266002D
quit
  221 2.0.0 Bye
  Connection closed by foreign host.

5. Check /home/herong/Maildir on the Postfix server to ensure the email is delivered correctly.

Ok. Both tests passed. I can inform others on my local network that my SMTP service is ready for email boxes under my domain name herongyang.com. They can use SMTP client tools to send emails to anyone who has a user account on my CentOS computer using email addresses in the format of <username>@herongyang.com.

As a reference, here a list of commonly used SMTP commands. All commands are case insensitive.

helo - Identify your client computer to the SMTP server. Example: helo 192.168.1.11
ehlo - Identify your client computer to the SMTP server and declare that you want to use ESMTP (Extended STMP) protocol. Example: ehlo 192.168.1.11
mail from:<address> - Specify the "From:" email address.
rcpt to:<address> - Specify the "To:" email address.
data - Start to enter the email subject and body. A new email will be delivered to the SMTP server when this command is terminated with a line that contains only a ".".
noop - Do nothing to keep connection open.
rset - Reset the session cache.
vrfy - Very the given email address on the SMTP server.
quit - Close the connection.