Exchange 2010 - Copy mailboxes from one company Exchange 2010 to another

We are merging with another company who use Exchange 365. We currently have Exchange 2010 on - premis. We will be creating a new AD users and new mailboxes, and need to get their current mailboxes into the new mailboxes. I believe we can export their current to PSTs, then import, but was wondering if there is a better solution.
mkllpitAsked:
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bobluisCommented:
The best way is staged migration for migrating from on-premises Exchange 2010 to Office 365. See the article for help:

https://support.office.com/en-us/article/Perform-a-staged-migration-of-email-to-Office-365-83bc0b69-de47-4cc4-a57d-47e478e4894e?ui=en-US&rs=en-US&ad=US
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