2576181 - Mail does not send out due to error "PKIX path building failed" when use gmail server

• Javamail has been configured to use gmail. And smtp.starttls.enable=true has been configured. 
• There are functionalities like BPM use javamail to send out notification mail. However at runtime, end user does not receive any mail.
• When check defaultTrace.*.trc log, following error can be seen:

Error sending notification email to recipient <mail address>. Reason: com.sap.bpem.base.mail.MailException: Cannot send e-mail because the e-mail session cannot be retrieved
......
Caused by: javax.mail.MessagingException: Can't send command to SMTP host;
nested exception is:
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
......
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:174)
at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:238)
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:318)
... 72 more

• SAP NetWeaver Java system

TLS, SSL, truststore, starttls, sslcertificate , KBA , BC-JAS-WEB , Web Container, HTTP, JavaMail, Servlets , BC-BMT-BPM-SRV , Process Server , Problem