Email migrations are very stiff and need to be taken care of during and post-migration. There are many tools available for migration purpose but it always depends on platforms from which you are migrating.
You can refer to one of the blogs where you come to things you need to take care of post-migration.
As you know post migration steps are important to avoid data loss similarly You also need to make sure you are ready for email migration before actually starting the migration . Learn more about things to consider for email migration.