Testing SMTP Using Telnet

Mark Berry August 17, 2007

I’m trying out a hosted anti-spam solution. This service works by becoming my public-facing mail server (DNS MX record), and relaying non-spam mail to the mail server in my office.

Due to a configuration error, I lost about 15 hours of email–the host wasn’t relaying for my domain. Email was bouncing with the message “553 Relaying is not supported”.

Lesson learned (with thanks to Ezra Herman):  before switching DNS, use Telnet to test the connection to the host and make sure it is accepting email for my domain. Here’s how to do it:

XFOR: Telnet to Port 25 to Test SMTP Communication
http://support.microsoft.com/kb/153119

The configuration is fixed, I sent myself an email using a Telnet session to the anti-spam host, so now I can update my DNS!

Update 4/28/2009:  This TechNet article explains testing SMTP with Telnet when authentication (but not SSL encryption) is needed:

How to Use Telnet to Test SMTP Communication
http://technet.microsoft.com/en-us/library/aa995718(EXCHG.65).aspx

Update 12/18/2009:  I tried to use Telnet to test sending a message to Go Daddy’s email server (smtp.secureserver.net). But whenever I typed

MAIL FROM:Admin@test.com

as described in KB 153119, I got this message:

501 #5.5.2 syntax error 'MAIL FROM:Admin@test.com'

I noticed in a MailEnable SMTP log that it encloses email addresses in angle brackets. Sure enough, when I typed

MAIL FROM:<Admin@test.com>

I got the expected response:

250 sender <Admin@test.com> ok