D-Link NetDefendOS User Manual page 536

alternatives for dropped email:
• A special email address can be configured to receive all dropped email. If this is done then
any TXT messages sent by the DNSBL servers (described next) that identified the email as
spam can be optionally inserted by NetDefendOS into the header of the forwarded email.
• If no receiver email address is configured for dropped emails then they are discarded by
NetDefendOS. The administrator can specify that an error message is sent back to the sender
address along with the TXT messages from the DNSBL servers that failed the email.
Tagging Spam
If an email is considered to be probably spam because the calculated sum is above the spam
threshold but it is below the drop threshold, then the Subject field of the email is changed and
prefixed with a message and the email is forwarded on to the intended recipient. The tag
message text is specified by the administrator but can be left blank (although that is not
recommended).
An example of tagging might be if the original Subject field is:
Buy this stock today!
And if the tag text is defined to be "*** SPAM ***", then the modified email's Subject field would
become:
*** SPAM *** Buy this stock today!
And this is what the email's recipient will see in the summary of their inbox contents. The
individual user could then decide to set up their own filters in the local client to deal with such
tagged emails, possibly sending it to a separate folder.
Adding X-Spam Information
If an email is determined to be spam and a forwarding address is configured for dropped emails,
then the administrator has the option to Add TXT Records to the email. A TXT Record is the
information sent back from the DNSBL server when the server thinks the sender is a source of
spam. This information can be inserted into the header of the email using the X-Spam tagging
convention before it is sent on. The X-Spam fields added are:
• X-Spam-Flag - This value will always be Yes.
• X-Spam-Checker-Version - The software that tagged the email.
• X-Spam-Status - This will always be DNSBL.
• X-Spam-Report - A list of the DNSBL servers that flagged the email as spam.
• X-Spam-TXT-Records - A list of TXT records sent by the DNSBL servers that identified the
email as spam.
• X-Spam_Sender-IP - IP address used by the email sender.
These fields can be referred to in filtering rules set up by the administrator in mail server
software.
Allowing for Failed DNSBL Servers
536
Chapter 6: Security Mechanisms