Mail projects at a minimum, and most others - entail a significant amount of change to users - some behind the scenes, some not. Please consider a feature that allows for notifications sent to users, and a UI to allow per project, or per customer level changes to those notifications.
At a high level I suspect customers would appreciate the following:
1- Welcome to the migration, or something similar. The goal here is to notify the user when an initial mail sync begins that their mailbox is part of a migration project. Effectively give them a heads up that things are happening in the background and to be on the lookout for future communications related to the migration. Customer level customization would allow for additional text such as "Contact the project team here, or your IT helpdesk here, if you have questions or concerns"
2- Cutover - this exists today, but as submitted seperately, there is no method to customize or brand this message today
3- Completion - similar to cutover, a welcome to your new tenant email might be useful to customers. Something that says hey, you've been moved from source to target and if you are reading this, you successfully completed your cutover. Customer level customization might provide instructions on performing a fresh password reset, steps to verify all of your email is there, and who to contact if there are issues or you suspect a problem. Additionally, this notification could serve as validation when changing over mobile or outlier devices, because this message would only exist in the target mailbox, not the source. If they don't see it, they didn't change over yet.
Domino to Exchange products from Binary Tree readily include communications, however Power365 seems very limited in terms of automated notification at various steps. Including some of these options leaves more room for migration administrators to customize the product to meet their specific business needs.