Permanently Clear Previous Mailbox Info

Thanks for the reply, but is that really the first step? Or I might have missed it?

I'm referring to https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites#:~:text=Before%20proceeding%20with%20either%20method%2C%20make%20sure%20that%20Mail%20Users%20have%20been%20provisioned%20for%20every%20user%20in%20the%20organization%20who%20will%20be%20migrated%20(either%20now%20or%20eventually).%20If%20any%20users%20aren%27t%20provisioned%2C%20provision%20them%20using%20the%20instructions%20in%20Manage%20mail%20users. which gives the following pre-requisites:

• https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites#create-a-subdomain-for-mail-routing-to-microsoft-365-or-office-365
• https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites#create-a-subdomain-for-mail-routing-to-your-google-workspace-domain
• https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites#check-google-cloud-platform-permissions
• https://learn.microsoft.com/en-us/exchange/mailbox-migration/googleworkspace-migration-prerequisites#provision-users-in-microsoft-365-or-office-365

The provision users section says:

Before proceeding with either method, make sure that Mail Users have been provisioned for every user in the organization who will be migrated (either now or eventually). If any users aren't provisioned, provision them using the instructions in https://learn.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-mail-users.

What's interesting is the paragraph that follows that one.

For more advanced scenarios, you may be able to deploy Azure Active Directory (Azure AD) Connect to provision your Mail Users. See https://learn.microsoft.com/en-us/office365/enterprise/deploy-office-365-directory-synchronization-dirsync-in-microsoft-azurefor an overview, and https://learn.microsoft.com/en-us/office365/enterprise/set-up-directory-synchronizationfor setup instructions. Then, you need to deploy an Exchange server in your on-premises environment for user management, and mail-enable your users using this server. For more information, see https://learn.microsoft.com/en-us/exchange/decommission-on-premises-exchangeand https://learn.microsoft.com/en-us/Exchange/recipients/mail-users. Once the Mail Users have been created in Microsoft 365, the Azure AD Connect may need to be disabled in order to allow the migration process to convert these users into mailboxes - see https://learn.microsoft.com/en-us/office365/enterprise/turn-off-directory-synchronization.

I hadn't thought of it before, but connecting the above paragraph with the scenario I mentioned and this blog post almost seems to suggest that the workaround is setup an Exchange on premise server! It would be nice if there was a better way of course; that's a lot of rigaramole.