Onprem Users have already an Office 365 Mailbox because of Teams trial phase

Copper Contributor

Dear Office365 pros.

We use per default Skype for business in our company but we already work with teams with a few users because other companies who use Teams, want that our emplyees also use teams to work together in some projects and we want to migrate all employees to Teams in the near future.

All this users in our company now have also a mailbox created in Office365 automatically
In these mailboxes are messages from teams and planer app only.

When we migrate to Teams, will these mailboxes merge with existing onprem mailboxes after we moved the onprem mailboxes with Exchange Hybrid Wizard to Exchange Online / Office 365 or how can i bring them together manually after Teams migration and Exchange Online migration ?


Thank you for your help

1 Reply

Hi @TomMoser,

Unfortunately no they won't merge and you'll also be unable to migrate any mailboxes to Exchange Online while these mailboxes exist. If you have Exchange Hybrid Configuration in place with AD Connect then these 'duplicate' mailboxes should not occur but you will also be unable to use calendars in Teams without the appropriate setup as per: https://docs.microsoft.com/en-us/MicrosoftTeams/exchange-teams-interact


I suggest looking at rectifying the duplicate mailboxes before they become a data loss problem by removing them following the steps outlined here:



If there is already vital data stored in these mailboxes, you can run an eDiscovery to capture and export the data before performing the deletion steps: