Using older versions of Outlook for Mac for personal email and calendar
Dear M.17,
Good day! Thank you for posting to Microsoft Community. We are happy to help you!
First if you are using outlook for Mac, please kindly try to login OWA(outlook web app) and then check whether you can reply these emails successfully.
Besides if everything is well in OWA, then please kindly try to rebuild a test profile on your outlook client to check if these problems will still occur, please kindly refer to it: Create a new profile. Before that, in order to prevent you from losing your data, please try to back up your data, please refer to it for more details: Export items to an archive file in Outlook for Mac
If you still need further help, please let me know and I'll do more research. Thanks in advance for your understanding! Your patience and cooperation will be highly appreciated. Hope you all the best!
Sincerely,
Kerry Chen | Microsoft Community Moderator