If you are a Zoho Mail user, you can find the affected services here. Accessing mail.zoho.com will redirect you to your Mail alone. For Calendar, please go to calendar.zoho.com. Other features like Tasks, Notes, Search etc will be available after the upgrade.
We will keep you updated on the progress. Thank you for your support.
UPDATE as on 2:40 am PDT: The infrastructure upgrade has been completed successfully. However, a few minor features within some services might be affected, we are working to restore these.
UPDATE as on 6:10 am PDT, on 2nd September 2013: All services and features are now fully operational. Order has been restored in the Zoho universe. Thanks for bearing with us through the maintenance period 🙂