Any time you change your MX records, it can take up to 72 hours for your new records to propagate. The duration varies depending on your hosting company and may be less than 72 hours; however, during this time, mail sent to your domain may bounce. While there's no way to avoid downtime entirely, there are several steps you can take to avoid bounces during the transition to Google Apps email.
Reduce MX record TTL in advance of changing MX records
Mail exchanger (MX) records contain a “Time to Live” (TTL) value that indicates how long Domain Name Service (DNS) servers should use the record before checking for updates. This is measured in seconds.
A TTL of 3600 seconds means records will take an hour to update. A TTL of 86400 means records will take a day to update. A higher TTL value means less traffic load for the DNS server, but it also means changing the MX records will take longer.
By reducing the TTL value in advance, you ensure servers aren’t using invalid records for too long when it comes time to switch the MX records to Google. To see how the TTL is listed, see Configure MX records. If you're still not sure how to reduce your MX record TTL, contact your domain host in advance to prepare for the switch.
Create all user accounts in Google Apps
To ensure that all mail to your domain is received after you change your MX records, make sure that you've created accounts in your Google Admin console for all existing users on your domain. If you have many existing email accounts, you can upload all the email addresses at once.
If you have any mailing lists or nicknames at your domain that may be affected when you change your MX records, you can recreate these in your Admin console. You may also wish to set up a catch-all address to handle messages sent to addresses that haven't been created at your domain.
Make the transition during an evening or weekend
To reduce the number of possible bounced messages when you change your domain's MX records, we recommend scheduling the change for an evening or weekend or other time when your email volume is low.
If your domain host has to input your new records for you, or if you're unsure of how to change your records, we suggest contacting your domain host in advance to prepare for the switch.
Notify key contacts of the change
To avoid confusion over any bounced messages, you may want to let some or all of your contacts know about the upcoming change to your email system.
Make sure to include the date and time of the planned change, instructions to resend any bounced messages, and any alternative contact channels people can use for time-sensitive issues. You can emphasize that any downtime should be brief, and that no messages will be lost during the transition; some may simply need to be resent.