Google Activation Guide Postini Email Security - Manual (Page 45)

FAQ: How MX Records Work 51
Every DNS host has a different user interface for MX records. Some use a trailing
period and some don't. Some allow you to set your TTL and some won't. Our
instructions include information for most common MX hosts, but yours may be
different. If you're not sure what to enter, use the same format as your existing MX
records. Be sure that the message security service MX records have the first
priority; the exact numbers don't matter as long as the message security service
MX records are the first. If your DNS server allows fewer than 4 records, enter as
many as you can.
For detailed instructions on how to update your MX records, see “Change Your
MX Records” on page 15.
Why do I need four separate MX records?
We use redundant MX records as a backup in case any problem occurs. They
give our network more flexibility if any changes to architecture are necessary.
Should I update A records when routing mail to the message security
service?
Do not update your A records. Since your MX records point to psmtp.com, you do
not need to change any A records.
Can I keep MX records pointed directly to my mail server as a backup?
Yes. If you want to keep a direct MX record for your mail server as a backup, you
can, but be sure to leave it at the lowest priority, after all four psmtp.com records.
Leaving your own mail server in the MX records should be a temporary backup
measure, because spammers sometimes try to bypass the service and connect
directly to your mail server using your backup MX records. Once your mail is
flowing through the message security service successfully, consider changing
your MX records to remove this backup.
How long do MX record updates take?
MX record updates are not immediate. After you've updated your MX records, it
will take a while for every sender to use the new MX records. Your original TTL
setting determines how long (in seconds) this will take. Changing the TTL setting
won't speed this up.
Will I lose mail after changing MX records?
No. As long as you enter the MX records correctly, you won't lose any MX records.
While your MX records are changing over, some mail will be delivered using your
old MX information, and some mail will be delivered using your new MX
information, but all of it will be delivered.