I'm receiving the following when I try to send email.  Email has been sending fine up until today.  Nothing has been changed on settings.  The domain ssl is valid.  It happens when trying to send to any customer.

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

Any help would be appreciated as we need to be responding to our customers.  Thanks!

Ryan