Cry Exchange How To...
Use telnet to test email (SMTP) delivery
You can use telnet (the command line utility) to test whether
your email (SMTP) server is receiving emails. You can also use it to test
that your server is not an
To send an email using telnet
- Open the command prompt
(Typically Start > run > cmd)
telnet mail.server 25
mail.server" is the name of your mail server. This can
either be the name of the server if it is on a LAN or its fully
qualified DNS name. For example
cryer.co.uk. The number 25 is
the port number that telnet is to connect on, this is always 25
for e-mail SMTP communications.
The server will acknowledge with a line that will identify
computer.name" is simply a name to identify your
computer. It can be anything you want or it can be blank.
MAIL FROM: sender.email
sender.email" is your email address. This is the address
that the email will appear to be from. You should get a response
starting with "250" and ending with "OK".
RCPT TO: recipient.email
recipient.email" is the email address you are sending to.
You should get a response starting with "250" and ending with
then type your message.
- To indicate the end of your message enter a full stop (".")
on its own on a line followed by pressing Enter.
- To close the telnet session enter:
Some gotchas when using telnet:
- Be aware that a backspace may appear to delete the character
to the left of the cursor, but in fact all it does is add a
backspace character to what you are sending. So when you type be
sure to be accurate and don't use backspace.
Being able to send emails using telnet provides you with a simple
- test that your exchange server cannot be used as an open
relay. Check that you cannot send an email (that is not destined
for your server) via your server from a location outside of your
- test a backup-email server without taking your primary email
server off-line - by connecting to it explicitly.
These notes have been tested with Exchange Server
2010 and Exchange Server
About the author: Brian Cryer
is a dedicated software developer and webmaster. For his day job he develops websites and desktop applications as well as providing IT services. He moonlights as a technical author and consultant.