Mimecast has a method to be able to replay emails to OnPremise Exchange which is neat 

We had a case needing to do this recently for a customer in 365. Mimecast tout their own product “Sync & Recover” for this however it was a one off thing and the extra cost couldn’t be justified

Thought of a way to do this without this , however you need to export the PST of individual users 

1. Export Mail to PST of each user missing mail
2. Reimport to PST to the office365 in the background for each user ( User the RootFolder in the CSV File as /Inbox