Cant send email - unable to find valid certification path to requested target

Cant send email - unable to find valid certification path to requested target

When im trying to send email from zoho through Java im getting the following error:

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

Everything worked fine a few days ago. and now its just not working.

What is the meaning of this Error??