Free & Busy Sync during Cross-Tenant Migration (Exchange Online)


Free & Busy Sync during Cross-Tenant Migration

You have two options “migrating” mailboxes cross-tenant:

-          Microsoft Cross-Tenant Migration
this will move, reconnect a user’s mailbox in target and removes the source mailbox entirely

-          Using a 3rd party tool, copying the mailbox content and syncing calendars
this will leave the source mailbox intact and can confuse users in the source environment, which user is now active (see the user adoption story)


Organization relationship in Exchange Online

If you migrate the User Mailbox entirely, meaning, nothing is left behind in the source, you can use Organizational Free/Busy Lookup:

This feature is named: Organization relationship in Exchange Online


This allows:

-          Set up an organization relationship to share calendar information with an external business partner.

-          Set up an organization relationship with another Microsoft 365 or Office 365 organization or with an Exchange on-premises organization.

But is not a free/busy synchronization, rather this is a normal EWS based calendar share and query feature. Users which are migrated, do not have a mailbox in the source tenant and there for a free/busy query against the target, with the new target email address can be initiated

There are three levels of access that you can specify:

·        No access.

·        Access to availability (free/busy) time only.

·        Access to free/busy, including time, subject, and location.


Follow this Microsoft article for setup:
Create an organization relationship in Exchange Online



It can be used while source mailbox are intact, but there must be a solution, e.g. Out of Office notification, stating other that a user has been migrated and is working actively on the target side !


Free/Busy sync cross-tenant with  Exchange Online


If you have used the copying mailbox content and permission option, e.g. with 3rd party tool. Mostly the source mailbox will stay as is. It might be required for accessing Share or Group mailboxes during the migration. If this is the case users still active in the source tenant need to know if a user is active where (source or target) if you use the Organization Relationship.

Another technology option comes into place, Calendar + Free/Busy Sync! It is always a 3rd party solution.

Free/Busy sync is established between the source and target tenant. Keeping users calendar in-sync. This applies to both possible directions from source to target and from target to source. If a calendar is in sync.


Free/Busy is part of a EWS calendar sharing feature and can be enabled e.g. in Quest on Demand cloud migration suite.



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