My previous company migrated from Exchange Server to Google Apps and we found Google's migration tools to work well and to be well-documented. They provide a to

My previous company migrated from Exchange Server to Google Apps and we found Google's migration tools to work well and to be well-documented. They provide a tool to do it at the server level https://support.google.com/a/bin/answer.py?hl=en&answer=172212 https://support.google.com/a/bin/answer.py?hl=en&answer=..., which sounds like what you would want, as well as a tool for individual users to do it themselves. Note that the server-oriented tool is named "for Microsoft Exchange" but it's actually for any IMAP server.