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X-Spam for Microsoft Exchange 2000/2003

Application Features

Mail Protocol Layer Checks
X-Spam performs a series of checks at the protocol layer to check for Spam. Figure 1 gives a screen shot from X-Spam with these features

Figure 1. Mail Protocol Layer Checks
(click on the image for bigger view)

The checks are explained below:

Real-time Black List (RBL)
An RBL is a DNS server that lists IPs of known Spam sending machines. RBL checking in X-Spam for Exchange 2000-2003 is carried out at the SMTP protocol level when the sending mail server connects to your mail server. If the IP is found to be in any one of the blacklists, the connection is terminated.

Dialup User's List (DUL)
A DUL is a DNS server that lists IPs of machines that are known to be part of the IP-range assigned to dialup Internet accounts. Spammers prefer using dialup accounts to send mail since they are easy and cheaper to sign up for and dispose. Most legitimate dialup user's usually connect to their ISPs mail servers or an Internet email account to send mail. The list of DULs is extensible just like RBL. If a connecting IP is found to be in any one of the blacklists, the connection is terminated.

MX/A DNS record verification
The domain part of the email address given in the "MAIL FROM" SMTP command is checked to see if it has a DNS MX (mail server) and/or A (IP) record.. Spammers or mis-configured client programs are the only ones who use non-existent domains in their emails. However, in either case it is absolutely valid to refuse acceptance of mail.

Reverse DNS
A reverse DNS check is performed to see if the connecting IP resolves to a valid domain name. Most properly configured mail servers usually have a reverse lookup (PTR) record entry in DNS. Further verification for the legitimacy of the resolved domain is done by checking if there is a sub domain match with the domain in "HELO/ELHO" and/or the "MAIL FROM:" SMTP commands. Since a majority of mail servers do not have a reverse lookup, this feature is disabled by default.

IP/Host/User Restriction From List

Whitelist: If an IP/Host/User is on this list, all mails from that IP or from that user/domain are accepted.
Blacklist: If an IP/Host/User is on this list, all mails from that IP or from that user/domain are rejected.

Host/User Restriction To List

Whitelist: If a Host/User is on this list, all mails to that user/domain are accepted.
Blacklist: If an Host/User is on this list, all mails to that user/domain are rejected.

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