If you click 'Yes' on a calendar invite in the Zoho Mail WEB app, the organiser/sender of the invite gets an email with subject: "Invitation Accepted : <original subject>" which then shows the organiser that you have accepted the invite in their calendar system.

If you click 'Yes' on a calendar invite the Zoho Mail Android App, that same email doesn't get sent at all.  When reviewing the invite in the web app, it shows the 'Yes' button as green (as though it has been clicked) so you can't re-click it and force a resend. 

To work around this, I've had a few calendar invites where I clicked 'Yes' in the mobile app, and the organiser has followed up wondering why I never responded.  It's confusing, because it shows I clicked 'Yes'.  The only workaround I've found is to update the status in the WEB app to say 'Tentative' and then 'Yes' again which then sends the emails and updates the organiser's calendar - but only from the WEB app.