Hi @ TA.Saurabh.Singh ,
In addition to adding an external domain, please follow these steps to check that your calendar is set up to allow users to share calendar details with external people:
1. In the M365 admin center, go to Settings > Organization Settings.
2. On the Services tab, select Calendar.
3. On the Calendar page, choose whether to allow users Microsoft 365 or Exchange to share their calendars with people outside your organization who have .
4. Choose what type of calendar information to make available to users. You can allow all information, or limit it to time only or time, subject, and location only.
Then you can share your calendar with an external user to see if you can share it busy/free Information.
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