Free/Busy (Calendar Availability) Not Working Between Multiple Domains

Hi everyone,

I’m running into an issue with Exchange Server where users from one domain cannot see the free/busy (calendar availability) status of users in another domain, even though both domains are part of the same Exchange organization.

Environment:

• Single Exchange organization (on-premises).
• Multiple accepted domains configured (e.g., http://domain1.com/ and domain2.com).
• All users are in the same organization, but their primary SMTP addresses belong to different domains.
• Free/busy works perfectly for users within the same domain.

Users from http://domain1.com/ cannot see free/busy information for users in http://domain2.com/ (and vice versa).

When manually granting 'Reviewer' permissions on the calendar to a user from another domain, everything works. But when it's only Free/Busy, it stops working.

Do I need to configure a federation trust and organization relationship even for multiple domains within a single Exchange organization? Most documentation talks about federation between separate organizations or hybrid setups, but not for this scenario.

If federation is required here, are there any special considerations or steps to follow? Or is there another approach to resolve free/busy visibility between domains in the same org?

Thanks in advance for any advice or shared experiences!