This white paper is a great start. Make sure to take the time to review your mail migration goals. Also, consider how mail should be moved. Note the details of your source and target systems, the number of mailboxes that will be migrated and the timeframe. Typical mail details include:
Cutover (one-shot/big bang) migration?A cutover mail migration is the simplest approach. It copies all mail in one step. A cutover migration is when all mailboxes are migrated over the weekend and then re-imported on Monday morning.
Staging migrationStaged migrations allow you to migrate mailboxes in batches. This allows you to transfer large amounts of data slowly to minimize impact on productivity. One business may choose to move certain departments first, or to migrate older mail over the weeks leading up to a final cutover when all of the recent mail has been migrated.
Are there archives that are still available?Are you using archives like Google Vault and Office In-Place archives already? Do these archives need to be archived? If so, are they an inbox, or a combination?
Are there new archives?Do you wish to archive any or all of your emails? If so, should they still be available to the user from their inbox. Office 365 Migration tool
Other items?Are there any additional items such as contacts and calendars, delegated tasks, journals, journals, notes, rules or shared mailboxes that should be migrated?