Technical data

Configuring and Managing SMTP
17.6 Configuring SMTP AntiSPAM
This example specifies the relay of mail from unknown SMTP clients to any
host within the def.com , abc.com ,or company.com domain. Because of implied
wildcarding, domains like VMShost.abc.com match against this list.
17.6.3.5 Rejecting Route-Through Attempts
If the SMTP server does not resolve the a route-through attempt using the
Good-Clients list and the Relay-Zones list, it rejects the RCPT TO command.
It allows an SMTP client to attempt route-through twice; if a third attempt is
made, the SMTP server rejects the RCPT TO command, disconnects the link, and
reports a SPAM event. For more information about SPAM event reporting, see
Section 17.6.8.
17.6.3.6 Examples of Specifying Good-Clients and Relay-Zones
In the following examples, host.abc.com is the host, and Good-Clients and
Relay-Zones lists are configured as follows:
Good-Clients: 1.2.0.0/16, 2.3.0.0/16, relay.abc.com
Relay-Zones: def.com, abc.com, company.com
The Good-Clients list specifies clients whose IP addresses are in the 1.2 or 2.3
subnets or whose IP addresses match the
relay.abc.com .
The following examples assume that
host.abc.com is not protected by a firewall
and has direct Internet connectivity.
1. The following example explains the process of handling a mail message where
the client is unknown and RCPT TO address is unknown.
A host with the IP address 2.2.3.5 connects to VMShost’s SMTP server. The
client sends a RCPT TO address of [e-mail] . The SMTP
server:
a. Fails to find a matching IP address in the Good-Clients list. The client is
considered unknown.
b. Fails to find the domain of the RCPT TO address in the Relay-Zones list.
c. The RCPT TO command is rejected with the following message:
<<<RCPT TO:<[e-mail]>
>>>550 User not local, Relay disabled.
2. This example shows the process of handling a mail message for which the
client is unknown but the RCPT TO address is accepted.
A host with the IP address 2.2.3.5 connects to VMShost’s SMTP server. This
IP address does not match Good-Clients, so the client is considered unknown.
However, if the client sends a RCPT TO address of
[e-mail] , the domain of the RCPT TO address is matched
against the Relay-Zones list. The RCPT TO address foobar.xxx.def.com
matches the Relay-Zones list, so the RCPT TO command is accepted.
3. In this example, the client with IP address 1.2.1.2 connects to VMShost’s
SMTP server. This IP address matches Good-Clients (it is in subnet 1.2).
Therefore, the client is considered known. The SMTP server does not check
the domains of the RCPT TO addresses.
Configuring and Managing SMTP 17–21