Security Validator problem

Security Validator problem

I use Zoho mail as my Email client....however in the last few days trying to send an email I get a Red Boxed warning that says:

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

The email does not send, nor end up in drafts.

I updated Java thinking it may have something to do with it...but no. I have no idea where to start on this problem.

Thanks