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Exchange 2010 cutover migration to Office 365

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..
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You have also option to copy PST files to hard disk and send it to Microsoft Datacentre
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Does the migration batch sync new email as they come in?  

Does it create all accounts in the migration batch first and then it syncs?  

If both are true I simply have it create the accounts, start the sync,  and once the sync is done change DNS.  I could even change DNS once the first sync cycle starts as long as the new mailbox is created and just let the mail sync.   The end users will be using OWA to check mail.
You are saying that, they are going to move to new domain and new domain profile
- Are you talking about new Web Domain and AD Domain ?
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Vasil Michev (MVP)
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I read some of the TechNet docs closely before seeing your response.. It sounds like the below steps will work..

Build the migration batch
Have it create the users and migrate the mailboxes
Let it run throughout the week
Once it seems to have finished the initial sync what to see the roll ups are catching up
Assign licenses on Office 365
Change TTL
Change MX record
Verify external mail is flowing
Leave sync running for in the event old DNS record result in old mail reaching the old mailboxes
Then stop/delete migration batch

The end users will be using OWA.. So.. No need to worry about Outlook profile right now..  Does this seems okay?  Also..  Once the leave the old domain, join the new one, and then log into the new domain profile..  I assume I need to add their Office 365 profile and pull the emails down?
The migration steps seem OK. Not sure how "changing the domain" part will work out, as they are many things to consider there. For starter, you will definitely want to verify the new domain in O365 and change the UPN and primary SMTP addresses to match it, in order to avoid user confusion and unnecessary login prompts.
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I'll probably use a utility to sync AD accounts from old domain to new domain.  I would imagine something is out there.  Then I would log into each windows profile on the new domain and manually add the Office 365 account to their Outlook profile.   Not sure if there is a more automated way of doing that.
There are some tools available for that part as well. My idea is that when you do this, you should aim to at least have matching usernames on-prem and in the cloud, which will make user's life much easier. Otherwise, expect to have to explain multiple times when to use which set of credentials. Anyway, one step at a time I guess, you should be all set for the email migration atm.
MigrationWiz is great and offers the desktop agent. I have a rep if you want contact info.
In the scenario of leaving a multi-tenancy Exchange environment to Office 365 I suggest using 3rd party tools..  The environment we're leaving from was MT and the native cutover migration wanted to move all mailboxes..  That or the admin was confused..  We ended up using a 3rd party utility..