Google Message Security for Google Apps Administration Guide - Manual (Page 175)

Spam Filters 175
3. For the org’s Default User (and any existing users), make sure the Filter
Status is On (go to Spam Filters on the user’s Overview page).
All obvious spam will be eliminated without reaching the data center or your
server. Any remaining spam detected by the filters is tagged with a spam
score written in the Header, and then delivered to users.
Configure Null Sender Disposition
Null Sender Disposition is an org-level setting on the Spam Filters page that lets
you choose how to dispose of messages that do not include an SMTP-envelope
sender address.
To configure Null Sender Disposition:
Select one of the following options:
• Ignore: Let the message bypass this filter. Other filters still apply.
• User Quarantine: Send the message to the recipient’s quarantine.
• Blackhole: Delete the message.
• Bounce: Return the message to the sender.
You can enter text to serve as the bounce message. If you enter text, it must
begin with 4 or 5, followed by two digits, a space, and your text. This structure
follows the format of SMTP reply codes. For example: 554 Transaction failed.
If you leave this field blank, the following message is used:
571 Domain does not accept delivery report messages
Note: In order to deliver valid messages that do not include an SMTP-envelope
sender address, like voicemail or vacation responders, use Content Manager to
create a custom filter. See “Create Filters to Allow Valid Null-Sender Messages”
on page 237 for more information.
Configure Null Sender Header Tag Validation
Note: These options are available only if you have been provisioned with
Outbound Services. If you configure Outbound Services for your mail server, then
the system adds a digital signature to each of your outbound messages.
Null Sender Header Tag Validation is the process by which the system examines
NDRs for the presence of an SMTP-envelope sender address and for the
message security service’s digital signature.
While this filter is an aspect of spam filtering, it runs at the very beginning of the
message filtering process to immediately dispose of messages like invalid NDRs.