Google Message Security for Google Apps Administration Guide - Manual (Page 259)

Approved and Blocked Sender Lists 259
How Senders Are Identified
The service identifies an approved or blocked sender by looking at the address in
the message’s From field. First, it looks at the From address shown in the
message header. If that is empty, it looks at the From address in the message’s
Envelope (which is typically hidden from view in email clients).
Reply-To headers aren’t checked because they aren’t necessarily assigned to the
actual sender.
How Domains Are Evaluated
When you include a domain in a sender list, any mail from that domain is
approved or blocked according to the list in which it is included. Approving or
blocking a domain affects all its subdomains, too. For example, if you approve
jumboinc.com, then you also approve sales.jumboinc.com and
marketing.jumboinc.com.
You cannot block a Top Level Domain (TLD) using the Blocked Senders list. A
TLD is the right-most part of the address, after the last period. Examples of some
common TLDs include
.com , .edu , .gov , .net , and .org (types of domains) and
locales, such as .uk , .mx , .jp , .ie , .ca . Instead, configure an IP range block on
the TLD. For details, see “Manual IP Block Configuration” on page 457.
Note: Sender lists use message and envelope headers to determine sender
addresses, and do not attempt to validate the sending email servers against
domain names (for example, through DNS lookups).
Editing Sender Lists in the Administration Console
You use the Administration Console to edit org-level and user-level sender lists.
Editing Approved/Blocked Senders for Organizations
By default, changes to the organization-level approved/blocked sender lists do not
retroactively propagate from an organization to sub-organizations. For example, if
you add an address to the approved sender list in an organization, this address
isn’t added to the sub-organizations.
There are character limits for sender lists at both the org and user levels. For more
information, see “Size Limits for Sender Lists” on page 262.
To edit org-level sender lists:
1. Go to Orgs and Users > Orgs.
2. Click the organization name in the organization list.
3. Under Inbound Services, click Sender Lists.