Response to invite not received by sender

Response to invite not received by sender

Hello - I received a meeting invite in my zoho email from an outside sender (office 365), and while I can select "Yes" on the invite, the sender does not receive my response. 
I have confirmed this with my own exchange hosted account on a different provider, by sending myself a calendar invite, and clicking "Yes". I never got a response to the invite. 

Please advise.