I have a website with a valid and trusted certificate that I'm connecting to. The root certificate authority GeoTrust Global CA is listed on Salesforce's list of outbound messaging CA's.
However, I'm still getting the certificate exception.
System.CalloutException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target