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July 26, 2013

Performing a Telnet on port 25 to simulate SMTP


When email servers are playing up and you need to find out what’s going on, sometimes it’s best to get your hand dirty. Use this Telnet trick to test the connection to the SMTP email server and find out what’s really causing the emails not to get through.

Telnet is a small command prompt style application that can be found in almost all Operating Systems including Windows XP, 7 and 8 plus most variants of Linux. Check out your OS’ manual for where you locate the program, it may not be installed by default.

Open a command prompt and enter the following command:

telnet servername 25

This opens a SMTP connection on port 25 to the mail server. Once connected, use the following commands to send an email.

RCPT TO: User@Domain.Com
Subject: test message


Once the SMTP server has received your instructions it will send the email as per your command. If there are any error messages, they will be displayed by the server within Telnet.

If there is a problem, you will receive a message, below are some examples of possible error messages:

  • insufficient resources:- this implies that the email server is low on memory, contact the server admin
  • Delivery not authorized, message refused:- the recipient of the email cannot be sent through this server. The server is probably locked down to stop it acting as a relay.
  • Mailbox full:- the recipient’s inbox is full

Further details can be here

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