Google Message Security for Google Apps Administration Guide - Manual (Page 171)

Spam Filters 171
Types of Spam Filters
When spam filtering is enabled for a user, the user’s messages are processed
through the following filters:
• If Blatant Spam Blocking is enabled for the user’s organization, the user’s
most obvious spam is bounced or blackholed (deleted), before it reaches your
email servers. This eliminates more than half of users’ spam, so neither you
nor they ever have to deal with it.
• Each user (and Default User) has a Bulk Email filter that sets a base level of
aggressiveness for filtering the remaining spam, which is typically sent to a
separate Quarantine for review.
• Each user (and Default User) can also optionally adjust four additional
Category filters to filter spam containing particular content even more
aggressively (sexually explicit content, special commercial offers, racially
insensitive material, or get-rich-quick schemes).
• Null Sender Disposition lets you choose how to dispose of messages that do
not include an SMTP-envelope sender address. These types of messages
are usually Non-Delivery Reports (NDRs). When the system receives an
inbound message, it checks for the SMTP-envelope sender address. If there
is no sender address, the message is disposed of according to the Null
Sender Disposition settings.
• Null Sender Header Tag Validation is the process by which the system
examines each inbound message for the presence of an SMTP-envelope
sender address and for the message security service’s digital signature. If
your message security service has been provisioned with Outbound Services
and you have them configured for your mail server, then the system tags the
Received field on outbound messages with a digital signature. When this filter
is on and the system receives an inbound message, it checks for the SMTP-
envelope sender address and for the digital signature. If there is no sender
address and the message doesn’t have the system signature, then the
message is disposed of according to the Null Sender Disposition settings. If
the system signature is present, then the message bypasses this filter, and is
evaluated by the others.
When Spam Filters Apply
Postini for Google Apps includes a layer of spam filtering that filters inbound mail
and quarantines messages with spam content. This filter is separate from the
Google Apps spam filtering that comes with email filters. This provides two distinct
layers of spam protection: Message Security, and Gmail spam filters.