Testing MX-SMTP with Telnet

by Chris Hall/December 1, 2016

Using your preferred Telnet client (such as putty), run the below list of commands:

telnet SERVERADDRESS 25
Note: 25 is the port for MX/SMTP

Once connected you will see something similar to the below:

SERVERADDRESS Microsoft ESMTP MAIL Service ready at Wed, 7 Aug 2013 11:30:12 +1000

At this point, type the below 2 commands into the Telnet console:

helo
mail from: external.address@contoso.com

You should either see a sent ok output or an error with an SMTP error code.
See below a link for a list error codes:
http://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml