Using MX Records and Spam Firewalls

Short Description
effectively using MX records in conjunction with a spam firewall, such … To effectively use MX records with spam firewalls, we recommend having the …

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Content
This whitepaper defines MX records and provides information about
effectively using MX records in conjunction with a spam firewall, such
as the Barracuda Spam Firewall.
MX Records ________________________________________________
MX records are DNS entries that are used by sending email servers to locate destination email
servers. An email server sending an email to a particular domain will look up the MX record for that
destination domain. The MX record provides a machine name or an IP address for the destination
domain. For example, if an email server wants to send an email to bob@mydomain.com, it would
perform an MX record look up on mydomain.com to determine the destination IP address. Once the
sending email server has the destination IP address, it would then be able to contact the
destination machine to deliver the email.
Using MX Records and Spam Firewalls
Multiple MX Records _________________________________________
Some domains have several MX records associated with it. Each MX record has a different priority
associated with it and each one points to a different server.
Figure 1: Basic MX Record Setup
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Figure 2: Domain with Multiple MX RecordsBarracuda Networks n Using MX Records and Spam Firewalls
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Using a Spam Firewall ________________________________________
To help block spam, some organizations may have their email server’s highest priority MX record
point to a spam firewall rather than the email server itself. This way the first machine to receive the
email would be the spam firewall. The spam firewall would then process the email and determine if
the email is legitimate. If it is, then it would forward the email to the destination email server.
To protect against the case of the spam firewall going down, some organizations have a lower
priority or backup MX record that points directly to the email server.
This, however, is not a recommended way to protect against a spam firewall failing. Why?
Spammers know about this method and will take advantage of the lower priority MX record that
bypasses the spam firewall. Spammers will send spam directly to the lower priority MX record so
that they will always bypass the spam firewall and get through to the email server.
Figure 3: Spam Firewall and MX Record
Figure 4: Wrong Method for Obtaining High Availability
When a sending email server performs an MX record lookup on…