Symantec 10744983 - Mail Security 8320, Administration Manual

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Third parties compile and manage lists of desirable or undesirable
IP addresses. These lists are queried using DNS lookups. When you
configure Symantec Mail Security to use a third-party sender list,
Symantec Mail Security checks whether the sending mail server is
on the list. If so, Symantec Mail Security performs a configured
action, based on the policies in place.
Incorporate lists
managed by other
parties
About Allowed and Blocked Senders Lists
The following sections provide important information about the Allowed Senders
Lists and Blocked Senders Lists.
Duplicate entries
You cannot have the exact same entry in both a Blocked Senders List and an
Allowed Senders List. If an entry already exists in one list, you will receive the
message “Duplicate sender - not added” when you try to add the same entry to
the other list. If you'd prefer to have this entry in the other list, first delete the
entry from the list that now contains it, then add it to the other list.
Similar entries
If you have two entries such as a@b.com and *@b.com in the two different lists,
the list with higher precedence “wins.”
See “About precedence” on page 83.
Performance impact of third party DNS lists
Incorporating third party lists adds additional steps to the filtering process. For
example, in a DNS list scenario, for each incoming message, the IP address of the
sending mail server is queried against the list, similar to a DNS query. If the
sending mail server is on the list, the mail is flagged as spam. If your mail volume
is sufficiently high, running incoming mail through a third party database could
hamper performance because of the requisite DNS lookups. Symantec recommends
that you use the Sender Reputation Service lists instead of enabling third party
lists.
Reasons to allow or block senders
Table 4-11 describes why you would employ lists of allowed or blocked senders
and lists an example of a pattern that you as the system administrator might use
to match the sender:
Configuring email filtering
Managing Email Firewall policies
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