Amer Networks E5Web GUI User Manual
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If dnsbl1 and dnsbl2 say an email is Spam but dnsbl3 does not, then the total calculated will be
3+2+0=5. Since the total of 5 is equal to (or greater than) the threshold then the email will be
treated as Spam.
If the Drop threshold in this example is set at 7 then all three DNSBL servers would have to
respond in order for the calculated sum to cause the email to be dropped (3+2+2=7).
Alternative Actions for Dropped Spam
If the calculated sum is greater than or equal to the Drop threshold value then the email is not
forwarded to the intended recipient. Instead the administrator can choose one of two
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 cOS Core into the header of the forwarded email.
•
If no receiver email address is configured for dropped emails then they are discarded by cOS
Core. 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
pre-fixed 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 will
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 cOS Core version that tagged the email.
•
X-Spam-Status - This will always be DNSBL.
•
X-Spam-Report - A list of DNSBL servers that flagged the email as Spam.
Chapter 6: Security Mechanisms
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