Hi Thilina,
Based on the description, the issue should be caused by the authentication of the federated user. Please do a Single Sign-On (SSO) test on www.testexchangeconnectivity.com. 
Then, please provide the entire test results (expand all of the test results) in the private message I’ve sent to you. You don’t need to hide the private information in the test result when replying to the private message so that we can troubleshoot the issue efficiently.
To see the private message, please refer to the following steps:
1. Go to "Your details" section on the right side of the community site.
2. Click Private messages.
3. Click the subject title <Configure Outlook for Mailbox Migrated users (on premise mail enabled users)> of the response to read the message.
Thanks,
Claud
