If an unprovisioned user had previously signed up for a consumer Google account (such as Gmail, Picasa, or Blogger) using the same email address they use in your domain, calendar invitations to and from that user on migrated users’ calendars will be associated with a conflicting account for the unprovisioned user (learn more about conflicting accounts). The only way to resolve this issue is to provision the user and then delete and recreate all events for which the user is the organizer or a guest.
In addition to provisioning user accounts in Google Apps, you need to complete the following steps before starting your migration:
Create nicknames for your users that match aliases they had on your mail server
Create groups that match the mailing lists on your mail server
Add domain aliases
For information about these configurations, see the Google Apps Administrator Help Center.
Configure Google Apps
Before you can migrate mail server data to Google Apps, you need to do the following in the Google Apps control panel:
Enable the Provisioning API (read only)
Configure OAuth Client
Preparing for Your Migration