Free & Busy Sync during Cross-Tenant Migration (Exchange Online)
You have two options “migrating” mailboxes cross-tenant:
this will move, reconnect a user’s mailbox in target and removes the source mailbox entirely
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)
If you migrate the User Mailbox entirely, meaning, nothing is left behind in the source, you can use Organizational Free/Busy Lookup:
Set up an
organization relationship to share calendar information with an external
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:
Access to availability (free/busy) time only.
Access to free/busy, including time, subject, and
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.
option comes into place, Calendar + Free/Busy Sync! It is always a 3rd
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