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While it is possible to construct a simple co-existence scenario using forwarding rules and the migration tool, this isn't actually something we suggest or support for Office 365. In the Office 365 environment, co-existence is really for on-prem Exchange environments. You'll need full admin rights and the ability to modify your local environment to create true co-existence, and that just isn't something you can do in a hosted environment.
Surely Microsoft want to encourage people to move - I am assuming this is the case because of MX records and the organisation of domain?
In this case it's actually a question of what you as a customer would like to accomplish. Due to the design of some of our services, in some cases you'll need to move, that's just how things are organized. In a Small Business subscription, for example, you need to fully redelegate your DNS records to Office 365 and can't really create a co-existence environment. For customers using our Enterprise subscriptions though, people frequently wish to create a co-existence environment due to the many unique advantages it offers, not the least of which is the ability to manage all of your users in the local AD rather than in the Office 365 administration tools. Really, it's a question of what sort of scenario you're working with and what your needs are.