A retention policy defines how long the Archive service should retain messages in the archive.
Administrators may define a single retention policy that applies to all messages, i.e. "retain all messages for seven years." Alternatively, an administrator may define multiple retention policies, with different retention periods for different messages depending on one or more defined aspects of those messages: sender, recipient, subject, body text contents, message size, presence of attachments, etc. This allows organizations the option to archive some messages but not others, or to keep different types of messages for different periods of time.