Thank you for posting your question in the Microsoft Q&A forum.
First, I apologize for the delay in response.
Can you please provide the exact build version on the affected user? For example:
You can try update your Outlook to version 2602 build 19725.20126 which was released on Feb 24th to see if it fixes the issue.
For the affected user, can they react on Outlook Web?
I have tested and the only time when I do not have the reaction options is when I use another user mailbox (delegation) or a shared mailbox. Can you please have a check at that as well?
If the issue happens on the primary account, you can try:
- Repair an Office application - Microsoft Support (Choose Repair Online)
- Create an Outlook profile - Microsoft Support(This doesn't affect your data)
- Lastly, reinstall Classic Outlook app.
I look forward for your reply.
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