give us email address policies
In OnPremise Exchange we have always used the email address polices to controle which email address users has, and what the default email address should be, OnPremise we use a field in AD like "department" to e.g. determine if the user should have .dk or .se or something else as default. it makes it easier when creating a new user and easier to handle if we need to change. please make same feature evailble in Office365, maybe just in powershell for a start?