Moving large numbers of emails between two MS Exchange (O365 and Outlook.com) accounts

I need to move all of my emails from an O365 account to an Outlook.com account. Both accounts can be accessed via the Exchange protocol. IMAP is disabled for the source, but is available for the destination.

Is there some clever way of doing this?

Manual (click and drag, or simple VBA based) transfers in Outlook do not seem at all reliable when there are 100000+ emails to move. The result is always Outlook getting confused, and me ending up restoring the emails removed from the source.

It would be good to have a way of doing the transfer "atomically". E.g. something that moves a single email, and then waits for both the source and destination to be sync'd before moving on to the next one.

Based on my experience, it's suggested that you could export all the emails of the O365 account as a .pst file, and then import the file into the outlook.com account. For more information about the import and export of emails, please refer to:

Export or backup email, contacts, and calendar to an Outlook .pst file Import email, contacts, and calendar from an Outlook .pst file

Hope to help you!