Exception in thread "main" java.lang.RuntimeException: javax.mail.MessagingException: Could not convert socket to TLS;
  nested exception is:
	javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
	at SendAttachment.main(SendAttachment.java:35)
Caused by: javax.mail.MessagingException: Could not convert socket to TLS;
  nested exception is:
	javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
	at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:2155)
	at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:752)
	at javax.mail.Service.connect(Service.java:388)
	at javax.mail.Service.connect(Service.java:246)
	at javax.mail.Service.connect(Service.java:195)
	at javax.mail.Transport.send0(Transport.java:254)
	at javax.mail.Transport.send(Transport.java:124)
	at SendAttachment.main(SendAttachment.java:32)
Caused by: javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
	at java.base/sun.security.ssl.Alert.createSSLException(Alert.java:131)
	at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:320)
	at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:263)
	at java.base/sun.security.ssl.TransportContext.fatal(TransportContext.java:258)
	at java.base/sun.security.ssl.CertificateMessage$T13CertificateConsumer.checkServerCerts(CertificateMessage.java:1324)
	at java.base/sun.security.ssl.CertificateMessage$T13CertificateConsumer.onConsumeCertificate(CertificateMessage.java:1199)
	at java.base/sun.security.ssl.CertificateMessage$T13CertificateConsumer.consume(CertificateMessage.java:1146)
	at java.base/sun.security.ssl.SSLHandshake.consume(SSLHandshake.java:392)
	at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:443)
	at java.base/sun.security.ssl.HandshakeContext.dispatch(HandshakeContext.java:421)
	at java.base/sun.security.ssl.TransportContext.dispatch(TransportContext.java:177)
	at java.base/sun.security.ssl.SSLTransport.decode(SSLTransport.java:164)
	at java.base/sun.security.ssl.SSLSocketImpl.decode(SSLSocketImpl.java:1152)
	at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1063)
	at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:402)
	at com.sun.mail.util.SocketFetcher.configureSSLSocket(SocketFetcher.java:626)
	at com.sun.mail.util.SocketFetcher.startTLS(SocketFetcher.java:553)
	at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:2150)
	... 7 more
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
	at java.base/sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:385)
	at java.base/sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:290)
	at java.base/sun.security.validator.Validator.validate(Validator.java:264)
	at java.base/sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:313)
	at java.base/sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:222)
	at java.base/sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:129)
	at java.base/sun.security.ssl.CertificateMessage$T13CertificateConsumer.checkServerCerts(CertificateMessage.java:1308)
	... 20 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
	at java.base/sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141)
	at java.base/sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:126)
	at java.base/java.security.cert.CertPathBuilder.build(CertPathBuilder.java:297)
	at java.base/sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:380)
	... 26 more