Table of Contents
This method tries to eliminate spam sent by e-mail servers connected
through Internet dial-up connections, as well as most ADSL and cable connections.
IP addresses of those connections are usually not registered to any DNS
as a qualified host meaning that they do not have their own static IP
and a registered host name like mail.domain.com.
A DNS lookup uses an Internet domain name to find an IP address, where
a reverse DNS lookup is using an Internet IP address to find a domain
name. Reverse DNS lookup technique is able to identify if the sending
e-mail server is legitimate and has a valid host name.
Many spammers use misconfigured hosts to disguise the source of the
spam. A DNS query that does not recover a matching host name and IP address
is a good indication that the message is spam.
DNS lookup is not always a good solution. Many legitimate e-mail servers
are incorrectly configured, or have intentionally not registered a name
with DNS, so a reverse query does not return a matching host name. Also,
this anti-spam method runs DNS queries on a large number or e-mails and
consumes valuable network resources. A number of problems, including network
delays and improperly configured networks or servers, can prevent legitimate
messages from getting through the filter. In January 2003, AT&T WorldNet
started using reverse DNS and was forced to remove the filter just 24
hours after it was deployed, after subscribers reported that messages
were going undelivered.
Ways to do DNS lookups
Reverse DNS lookup
This method is time-consuming and it is rarely used. The receiving server
performs a reverse DNS lookup on the IP address of the incoming connection
and checks if there is a valid domain name associated to it.
The receiving server will get the host name of the sending e-mail server
from the SMTP HELO command, perform a simple DNS query (forward DNS lookup)
and verify that the IP address is indeed the IP address of the incoming
connection. If the resulting IP address does not match the incoming connection
IP address (sender's IP address), e-mail is rejected.
Sender's address lookup
When ISPs check whether an incoming e-mail is accepted, they can do
a DNS check on the sender's e-mail address. For example, if your address
is , then the ISP does an nslookup on domain.com. If no
records are found - the message is rejected.
A variation of this method is checking if there is an MX DNS record
of the domain.com. MX record returns an address like mx1.domain.com used
to connect to the server that accepts messages for domain.com. Even if
the domain in the sender's e-mail address is valid, but there is no e-mail
server for domain.com - the message is not accepted.
The solution depends on which method is used to block spam.
1. Reverse DNS lookup
Get a domain name
To get a domain name for your dynamic IP
address you can use the no-ip.com DNS service which enables you to host
a server using a dynamically assigned IP address. When you send messages,
if any of ISPs perform a reverse DNS lookup of your IP address, they will
always get a valid domain name and accept messages sent from your computer.
The basic service is free, but the names
are sub domains of names already registered by No-IP like: "servequake.com"
or "myvnc.com". For more information, visit this web page:
No-IP Plus enables you to use your own, separately
registered domain name. The price for one year is $24.95:
Use backup SMTP servers
Edition of PostCast Server has a feature that allows you to specify
one or more backup SMTP servers. If only certain domains are unable to
receive messages from PostCast Server, you can use this option to forward
those messages to your ISP's SMTP server. Open the Settings/Undelivered/Gateways
window to configure this feature. For more information, see SMTP
Use socks proxy servers
This feature enables you to relay e-mail
through other servers. When the message is sent using a third-party socks
proxy, your IP address does not appear as the source of the message. The
best solution is to connect to your ISP's socks proxy directly if it is
provided by the ISP. Their server's (non-dynamic) IP address will be the
source of your outgoing messages. For more infromation, see Firewall
and Proxy Support.
2. Sender's address lookup
Make sure that e-mail address in the From
field of your messages is always valid.
3. HELO lookup
AOL, Hotmail, Yahoo, and some other ISPs
perform a HELO lookup when receiving messages. If the lookup is not successful,
they simply reject to deliver the message to the recipient without sending
any error message. There are three possible ways to solve this problem.
1. You can select the "Resolved Internet
IP" option in the HELO handshaking
settings in the Settings/Advanced screen. The program will perform
a DNS query to find out which address points to your IP. This option sometimes
does not return the correct values if you are behind a router. If that
is the case, you can use the http://network-tools.com/
service to check your IP address and look for "Host name" which
should then be copied into the "Use this Identification" box
in HELO handshaking settings.
2. Try to change the server identity in the
HELO handshaking settings in the Settings/Advanced screen to the "mail.domain.com"
format. For example, if your ISP provides e-mail address such as ,
set the HELO handshaking identification to mail.domain.com. Try also with
only 'domain.com' format.
3. If you have a domain name that points
to your computer's IP address, then enter that domain name in the HELO
handshaking settings in PostCast Server. You can use the no-ip.com service
to host a domain name on your computer.
- Internet Black and White Lists
- DNS Lookups