iOS certificate problem

Hello Zoho team,
Since last week I continuously getting the certificate error from my iOS devices that claims - certificate cannot be checked. That happens with your mail. I using exchange mobile sync through iOS mail app for email only - contacts, calendars, notes etc. are synchronized with my iCloud account not with zoo and never had some issues but last week. Error is that the certificate cannot be checked/validated and when I choose option for more details I getting that certificate that can't be validated. I made a screenshot which I attaching here too. Mail works though but still the error is present... I'll forward that mail to your support team too. Home this issue will be solved soon....
Thanks and happy new year guys :-)