Exchange Server 2003 to Office 365 Cutover Migration

I have a Exchange Server 2003 with 22 mailboxes and we are going to Outlook 365. I just want to make sure the Cutover Migration is what I want to do. There seems to be a couple things I couldn't find answers to.

1) They call it syncing. Is it a true sync? So if mail keeps coming in to the Exchange Server during the migration will that mail be synced to Office 365?
2) On the flip side if mail comes in to Office 365 during the migration will it be synced back to the Exchange Server?

My concern is losing emails during the transition and when to change the MX records. Any help?
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LockDown32OwnerAsked:
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Mark GalvinManaging Director / Principal ConsultantCommented:
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LockDown32OwnerAuthor Commented:
That was part of the question but even the link you posted doesn't cover the questions above. What is the best approach and is there a detailed list to follow? Like:

1) Leave MX records pointing to local Exchange Server
2) Do Cutover migtation
3) Change MX record to....

My concern is losing emails in the transition because it doesn't explicitly sate what happens.....
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Mark GalvinManaging Director / Principal ConsultantCommented:
You set the Office 365 in Hybrid mode. Change MX Records to Office 365. Mail comes into Office 365 and if the user doesn't have an Office 365 mailbox (I.E. not yet migrated) then the email goes to the on premise server.  

Thats the simplest way to explain it.

Very good article here http://www.msexchange.org/articles-tutorials/office-365/exchange-online/configuring-exchange-hybrid-deployment-migrating-to-office-365-exchange-online-part1.html
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Vasil Michev (MVP)Commented:
Cutover will do incremental syncs every 24h after the initial one, so messages will arrive, albeit with a delay. There is no sync back to the on-prem server however.

In any case, for so few mailboxes you should be able to do the migration overnight, so I wouldnt worry too much about it.
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LockDown32OwnerAuthor Commented:
OK. That is what I gathered from the information I could find. So you can just let the migration run while changing MX records and stuff and anything that straggles in to the Exchange server will eventually show up in Office 365. Thanks!
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