Hi @Pat Zagelow ,
Thanks for your reply!
Based on my test in my Outlook 365, after i replied to the emails successfully, the Reply icon will be displayed. I found that if i tried to reply to an email but the replied email is not sent actually for some reason (although it appears in my sent folder), then i received an NDR email and the Reply icon will not be displayed. Please check if you have replied to the emails successfully.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.