Our users are getting this message when trying to send e-mails as usual from Zoho. 

All has been perfectly fine up until now so this is a new issue. Using same accounts in Outlook and WebMail all OK so seems to be a Zoho specific issue.

Please advise urgently if possible?

Reason:sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target