Let’s look at the conversation again.

HELO noddy.com                                                       

who I am- email host/domain,

MAIL FROM: dude@noddy.com                               

who I am – email address

RCPT TO: anotherdude@target.local                          

who I am mailing. Multiple recipients may be specified by using multiple RCPT lines 

DATA 

body of the message, including any further arguments, such as subject lines. This needs to be ended using a period “.” Followed by an empty line. Notice that extra tags such as subject: need to be followed by an empty line.

What we have achieved here is test that SMTP is up and  accepting mail. To make this more interesting, test for two way communication by requesting a delivery receipt:

Change the RCPT TO line to read:

RCPT TO :anotherdude@target.local notify=success,failure

How would we test for other conditions such as relaying allowed? Simple – change the RCPT TO: line to read another SMTP address for which delivery is not enabled. Here’s an example

If your mail host is configured correctly you will receive a polite – or not so polite message saying that your mail cannot be accepted. If the opposite occurs and your mail submission is succesfull for an email domain that doesn’t isn’t part of your SMTP servers inbound domain list – you have an open relay. A SPAM target. If this is facing the internet you should fix it really soon. Like now.

Here is a basic list of commands. This list is extensive enough to test anything you’re willing to try without breaking out of Telnet and using a mail client. These were sourced from RFC 821, which may be found here: http://www.faqs.org/rfcs/rfc821.html

Some of these are not discussed above.

Common Messages are listed below, however - to get to the exact meaning of virtually any SMTP code, have a look here: http://www.faqs.org/rfcs/rfc1893.html