If a meeting has been created with any field (Subject, title, or message body) empty, then the recipient will receive the automated error message in reply that their "response was not delivered" after responding to the meeting invitation. However, this may not affect their response and their response may still be recorded. Also, if they have accepted the meeting using their phones, this issue might occur.

In such cases, it is always recommended to confirm with the recipient after, if they have received your response.