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What
is mail relay?
Mail relaying occurs when mail is forwarded from someone else through your mail
server. Forwarding mail through a mail server is perfectly
natural as long as either the originator of the message or
the receiver is a local user. If neither the originator nor
the receiver of the message is a local user, it is called
third-party mail relaying. Usually the mail servers can be
configured not to accept third-party mail relay, but often
they are not. Mail servers that allow third-party mail
relaying are of special attraction to "mail spammers" for
sending large amounts of junk e-mail under false
identity.
How is it
done?
Mail relay can best be demonstrated with an example. In
this example, both the sender and the recipient are outside
the local domain. The mail server is from an entirely unrelated third
party to this transaction. The message really has no
business passing through this server.
Source: http://maps.vix.com/tsi/ar-what.html
Since many network administrators have blocked out or
filtered messages from known spammers, spammers have had to
develop new techniques to evade the blockades. Spammers,
therefore hijack third-party mail servers to get their junk
e-mail through the spam filters. Another purpose seems to be
to increase the number of messages they can spew, all at no
cost to them. This is a theft and it damages the resources
of those unfortunate companies that are the victims.
What are the
consequences?
Crashed or damaged mail system Mail systems of the
companies that suffer from third-party mail relay may crash
or get damaged, and they might even lose valuable data.
Extraordinarily high recovery cost The companies
will have to spend unplanned persons-hours to recover from
the crisis.
Blacklisted and denial of service The worst
consequences are that companies that suffer from third-party
mail relays, will often be blacklisted by other organizations
on the Internet and therefore they might be cut off from
various services they need.
Overall results are lost credibility and tarnished
reputation
Links
for further information on mail relay
Read about DNS spoofing here
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