In such a migration you may experience the following problem, your DNS zone is configured as:
exchange.domain.tld A 3600 10.0.0.1
where 10.0.0.1 is the IP address of your Microsoft Exchange instance.
While you are migrating to Google For Work, you will probably want to keep online your Microsoft Exchange instance as a backup, in case you need to go back, or for the main reason that you’ll need it to move along your INBOX, folders and Contacts of thousands of accounts.
However, this DNS record will be conflictual when you’ll want to sync an Iphone or any hardware using IOs using the native mail application : when following Google procedure that ask for selecting « Exchange » account on your Iphone, the Iphone will look for exchange.domain.tld in your DNS zone and connection will fail.
As a workaround, you can simply follow the google procedure, but as a first attempt, give a wrong password. You will then be able to configure the whole details, including the Google server address.