Google Message Archiving Administration Guide - Manual (Page 26)

Introduction 27
The process by which users’ email enters the archive differs, however, depending
on whether you choose to use inbound/outbound archiving or journal archiving.
Message Capture for Inbound/Outbound Archiving
If you use inbound/outbound archiving, rather than journal archiving, Message
Archiving automatically captures, stores, and indexes legitimate inbound and,
optionally, outbound messages as they flow through your message security
service. The following figure shows an overview of how Message Archiving
captures inbound and outbound messages.
How Inbound Messages Are Archived
When the message security service receives an inbound message, it filters the
message if the recipient is a registered user (has an account on the service).
If the message for a registered user is not blackholed, bounced, or quarantined as
a result of filtering, or if the recipient is not a registered user, then the service
attempts to deliver the message to your email server. If your email server confirms
that the recipient is valid, it sends a 250 response code to your message security
service.
If the recipient is valid and has an account on your message security service,
Message Archiving archives the message only if the account resides in a user
organization for which archiving is turned on. (You can determine which users in
your message security service have messages archived by assigning them to
different organizations.)
If the recipient is valid and messages are archived for that account, but your mail
server rejects the message for some reason after it was filtered by the message
security service, then the message is still archived even though it was never
delivered to the recipient. In this case, the message security service relays the
response from the recipient server to the sending server.
If the recipient is valid but does not have an account on your message security
service, Message Archiving does not archive the message.