SMTP Protocol level

The table below contains a summary of a complete SMTP message transfer session between the sending MTA, tx-moon.com, and the receiving MTA, rx-earth.com. In this conversation the sending MTA is written in bold blue characters while the receiving MTA is in normal green characters, and comments are in italics.

Sender or Receiver	Comments
	Sender tx-moon.com connects to port 25 at Receiver rx-earth.com and session is established
220 rx-earth.com ESMTP Sendmail 8.12.7/8.12.6; Mon, 10 Mar 2003	rx-earth.com announces that it is listening
HELO tx.moon.com	Sender identifies itself
250 Pleased to meet you	Receiver returns greeting
MAIL FROM: <sender@tx-moon.com>	Sender identifies the message sender's address
250 OK	Receiver confirms sender address
RCPT TO: <recipient@rx-earth.com>	Sender gives the address of the message recipient
250 OK	Receiver confirms the address
DATA	Sender states that it is about to transmit the message
354 Enter mail, end with CRLF.CRLF	Receiver states that it is ready to accept the message
message data transmission begins : : : .	Sender starts sending the message which includes the message header, body text, and any attachment(s) Period on line by itself indicates the end of the message data
250 OK	Receiver states that it got the entire message data
QUIT	Sender states that it is done
	Receiver closes the connection

The Microsoft SMTP virtual server provides connection based filtering by using the IP address of the connected session, even before any greeting is exchanged. These include the following.

Unfortunately this leaves out a very useful filtering technique that can eliminate a substantial amount of spam — the ability to check various DNS-based blacklist servers to see if the connected IP belongs to known spammer machines. Praetor G2 provides this missing filter

Note:

If you were using the previous version 1.5 of Praetor, you will note that many other SMTP protocol level tests appear to have been removed. In reality, these facilities have been moved to message-level testing resulting from this G2 version's reliance on the Windows 2000/XP SMTP virtual server.

      1. MAIL FROM sender address check
    2. MAIL FROM sender domain check
    3. RCPT TO recipient address check

Naturally, this will require the message to be received in its entirety instead of abruptly terminating the message transfer process by disconnecting the session.

Order of SMTP protocol level tests

The order of these tests are as follows.

Banned IP blacklist
Reverse DNS check on domain name
Relay check
DNS blacklist filter