Technical data

Configuring and Managing SMTP
17.6 Configuring SMTP AntiSPAM
17.6.4 Blocking Mail from Specified Clients
You can configure the SMTP server to automatically reject any mail transactions
with specified SMTP clients. To enable this feature, configure the Bad-Clients
list in SMTP.CONFIG. The syntax of the Bad-Clients list is the same as the
Good-Clients list. For example:
Bad-Clients: 1.2.3.5, 100.101.102.103
If Bad-Clients is configured, the SMTP server checks the IP address of the client
against the list. If a match occurs, the SMTP client is considered ‘‘known bad;’’
the server sends a failure message to the client and then disconnects the link.
17.6.4.1 Resolving Conflicts between Bad-Clients and Good-Clients
The Bad-Clients and Good-Clients lists are not mutually exclusive. If an SMTP
client’s IP address may be resolved in both lists, the entry that most closely
matches the client’s IP address is used.
For example, the following lists are configured:
Bad-Clients: 1.0.0.0/8
Good-Clients: 1.2.3.6
When an SMTP connection comes in from IP address 1.2.3.6, which is in the
1.0.0.0 subnet, the client may be considered a known bad client. But because the
specific IP address is specified in the Good-Clients list, the message is accepted.
In a case where the exact IP address is specified in both the Good-Clients list and
the Bad-Clients list, the SMTP server accepts the message.
17.6.5 Real-Time Black Hole Lists (RBL)
To prevent SPAM route-through from clients dialed through an ISP and
connecting directly to the destination hosts, the Internet community maintains
a list of known SPAMming IP addresses at domain rbl.maps.vix.com . This is
called the Realtime Blackhole List (RBL) and contains DNS A records.
To determine whether a specific IP address is in the RBL list, perform these
steps:
1. Reverse the IP address.
2. Append the domain of the RBL.
3. Do a DNS A record lookup.
For example, to check the IP address of 2.2.3.5, do a name lookup of
5.3.2.2.rbl.maps.vix.com . If the query returns something, then IP address
2.2.3.5 is in the rbl.maps.vix.com SPAMmer list. All RBLs are implemented this
way.
To take advantage of the RBL, configure the RBLs list in SMTP.CONFIG. The
RBLs configuration option lists the domains providing RBL services. You can
specify a list of RBLs, thereby accommodating individual RBLs and additional
Internet-provided RBLs along with the current one.
For example:
RBLs: rbl.maps.vix.com, rbl.ourcompany.com
If the SMTP server matches the IP address of the client with an entry in any
of the RBLs in the list, the server sends a failure message to the client and
disconnects the link.
17–22 Configuring and Managing SMTP