Google provides an Administration Guide for email migrations to Google Apps. Google has done a great job of documenting the migration process and even provides a Google Apps Migration for Microsoft® Exchange tool as well as video documentation.
You download this tool, provide a user list, and the tool will authenticate to your existing email server and migrate the email to Google Apps platform.
The format for the user list is as follows:
user1#user1password, google_user1
For example, if you had a user with the email address of [email protected] and a password of “letmein” then your user list would be:
[email protected]#letmein, migration
Enjoy!