Manually testing SMTP mail issues via Telnet


So this may be a post that exists in a million places on the web if you know where to look, however it would appear that whenever i do this a colleague gets the idea that i am channelling black magic. How does one “debug” a mail server connection – this is as easy as 123.

email-error-missing This can be a complete life saver in times of need. Maybe you’re trying to get your site to send mail to a server that is not allowing relaying? Maybe you’re setting up a new mail server or debugging an old one?

Who knows, who cares, lets get to it.


1. Open a command window (we’re in windows… yes this may upset you, no i don’t care)

2. Type:

telnet 25


Simply follow the conversation below to send a test email:

You type The server responds

Telnet {} 25

220 *Indentifying details*


250 *message*

MAIL FROM: {sender address}

250 is syntactically correct

RCPT TO: {address to send to}

250 is syntactically correct
550 relaying denied

DATA Tells you to send data
Subject: {your subject} ENTER
your message

250 (message saying its added to queue)

QUIT (to disconnect) A signoff message

Potential Errors

The conversation above may not play out exactly as described but it will definitely indicate if there is any issues from the clients’ point of view. If you receive messages about relaying being denied etc, you will quickly have a new avenue to investigate – this can be your silver bullet.