SMTP TLS handshake fails

[dokterbob]

When attempting to use an external SMTP server, a certificate validation exception is raised:

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
   at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1420)
   at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1408)
   at com.sun.mail.smtp.SMTPTransport.ehlo(SMTPTransport.java:847)
   at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:384)
   at javax.mail.Service.connect(Service.java:297)
   at javax.mail.Service.connect(Service.java:156)
   at javax.mail.Service.connect(Service.java:105)
   at javax.mail.Transport.send0(Transport.java:168)
   at javax.mail.Transport.send(Transport.java:98)
   at net.appjet.ajstdlib.email$.sendEmail(ajstdlib.scala:247)
   at net.appjet.ajstdlib.email.sendEmail(ajstdlib.scala)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.lang.reflect.Method.invoke(Method.java:606)
   at org.mozilla.javascript.MemberBox.invoke(MemberBox.java:161)
   at org.mozilla.javascript.NativeJavaMethod.call(NativeJavaMethod.java:247)
   at org.mozilla.javascript.optimizer.OptRuntime.callN(OptRuntime.java:86)
   at JS$module$email$js$23._c_sendEmail_3(module email.js:86)
   at JS$module$email$js$23.call(module email.js)
   at org.mozilla.javascript.optimizer.OptRuntime.callName(OptRuntime.java:97)
   at JS$module$email$js$23._c_sendEmailLoggingExceptions_5(module email.js:96)
   at JS$module$email$js$23.call(module email.js)
   at org.mozilla.javascript.optimizer.OptRuntime.callName(OptRuntime.java:97)
   at JS$module$email$js$23._c_anonymous_2(module email.js:56)
   at JS$module$email$js$23.call(module email.js)
   at org.mozilla.javascript.ScriptRuntime.applyOrCall(ScriptRuntime.java:2521)
   at org.mozilla.javascript.BaseFunction.execIdCall(BaseFunction.java:300)
   at org.mozilla.javascript.IdFunctionObject.call(IdFunctionObject.java:129)
   at org.mozilla.javascript.optimizer.OptRuntime.call2(OptRuntime.java:76)
   at JS$module$onscheduledtask$js$261._c_anonymous_1(module onscheduledtask.js:32)
   at JS$module$onscheduledtask$js$261.call(module onscheduledtask.js)
   at org.mozilla.javascript.optimizer.OptRuntime.call0(OptRuntime.java:57)
   at JS$module$onscheduledtask$js$261._c_script_0(module onscheduledtask.js:17)
   at JS$module$onscheduledtask$js$261.call(module onscheduledtask.js)
   at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:426)
   at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3178)
   at JS$module$onscheduledtask$js$261.call(module onscheduledtask.js)
   at JS$module$onscheduledtask$js$261.exec(module onscheduledtask.js)
   at net.appjet.bodylock.InnerExecutable$$anonfun$execute$1.apply(bodylock.scala:76)
   at net.appjet.bodylock.InnerExecutable$$anonfun$execute$1.apply(bodylock.scala:75)
   at net.appjet.bodylock.BodyLock$.runInContext(bodylock.scala:111)
   at net.appjet.bodylock.InnerExecutable.execute(bodylock.scala:75)
   at net.appjet.oui.execution$$anonfun$execute$4.apply(execution.scala:517)
   at net.appjet.oui.NoninheritedDynamicVariable.withValue(dynamicvar.scala:38)
   at net.appjet.oui.ExecutionContextUtils$.withContext(execution.scala:340)
   at net.appjet.oui.execution$.execute(execution.scala:513)
   at net.appjet.oui.execution$.runOutOfBand(execution.scala:640)
   at net.appjet.ajstdlib.execution$.runTask(ajstdlib.scala:140)
   at net.appjet.ajstdlib.execution$TaskRunner.call(ajstdlib.scala:177)
   at java.util.concurrent.FutureTask.run(FutureTask.java:262)
   at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:178)
   at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:292)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
   at java.lang.Thread.run(Thread.java:745)
Caused by: 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
   at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
   at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1904)
   at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:279)
   at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:273)
   at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1446)
   at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:209)
   at sun.security.ssl.Handshaker.processLoop(Handshaker.java:913)
   at sun.security.ssl.Handshaker.process_record(Handshaker.java:849)
   at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1023)
   at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1332)
   at sun.security.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:709)
   at sun.security.ssl.AppOutputStream.write(AppOutputStream.java:122)
   at com.sun.mail.util.TraceOutputStream.write(TraceOutputStream.java:101)
   at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
   at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
   at com.sun.mail.smtp.SMTPTransport.sendCommand(SMTPTransport.java:1418)
   ... 55 more
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
   at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:385)
   at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:292)
   at sun.security.validator.Validator.validate(Validator.java:260)
   at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:326)
   at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:231)
   at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:126)
   at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1428)
   ... 66 more
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:196)
   at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:268)
   at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:380)