Google Activation Guide Postini Email Security - Manual (Page 11)

Change Your MX Records 15
Change Your MX Records
Once you complete the Setup Wizard, you receive an activation email, which
provides the information you need to change your mail exchange (MX) records.
The activation email arrives within two business days of completing the wizard.
MX records represent the mailing address for your domain's email. To ensure that
messages for users within your domain are filtered and/or archived, you need to
change your domain’s MX records to point to the message security service.
To change your MX records, you must first log in to your domain registrar.
To do this, follow the instructions below.
IMPORTANT: Don’t change MX records for any domain other than the one listed
in your activation email. You can add other domains to the service and change
their records later.’
Note that MX records may take 48 hours or longer to propagate after you change
your records to the message security service, although propagation may take just
one hour in some cases. The length of time that’s required for propagation
depends on the Time to Live (TTL) setting that was previously specified for your
domain. During this transition, your mail will be delivered as usual, but not all mail
will be filtered by the message security service until your records have fully
propagated.