I'm considering the cutover migration option for migrating from on premise Exchange 2010 to Office 365. It seems pretty straight forward, however, during the time of the migration I would imagine user's mailboxes will be not accessible.. I plan to migrate to Office 365 and have the users use OWA to access their email for a few weeks to void using their Outlook clients.. The reason being their current domain, domain profile, and on premise Exchange server will go away.. They'll be moving to a new domain and new domain profile a few weeks after the migration. Then I would create an Outlook profile on their new domain profile and pull down their mail via a department at a time.. So.. Back to my original concern.. It might take 3 or 4 days to migrate 300 GBs of emails... Their internet circuit is 100mbps... But it's so hard to know how long it will take. I'm trying to void email downtime on the weekdays..