Hi @Dell Dell ,
Agree with AshokM that there could be minimal downtime during the mail flow cutover procedure which involves DNS change. So it's recommended to do the mail flow cutovers after hours if you are converned about the potential impact.
Besides, before introducing Exchange 2016 into the existing environment, please make sure the following co-existence requirements are met:
- Exchange Server 2010 has been fully updated to support the coexistence which requires SP3 with Update Rollup 11 at a minimum. You can download the latest Update Rollup 30 for SP3 and apply the latest security update KB5000978 as well.
- In Exchange 2016, the minimum support Forest Functional Level and Domain Functional Level is Windows 2008 or above. So please ensure your environment is ready for this.
- The Outlook clients are upgraded to Outlook 2010 or above on Windows and Outlook 2011 or higher on the Mac.
As regards to the migration steps, the Exchange Deployment Assistant tool shared by AshokM is highly recommended. It asks you a few questions about your current environment and then generates a custom step-by-step checklist for you to deploy Exchange server. Hopefully you can find it helpful.
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