Basically, don't know nothing about certificates, i'm getting this error ( windows 7x64, java 1.8 x64).

Exception in thread "main" javax.mail.MessagingException: sun.security.validator
.ValidatorException: PKIX path building failed: sun.security.provider.certpath.S
unCertPathBuilderException: unable to find valid certification path to requested
 target;
  nested exception is:
        javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorExc
eption: PKIX path building failed: sun.security.provider.certpath.SunCertPathBui
lderException: unable to find valid certification path to requested target
        at com.sun.mail.imap.IMAPStore.protocolConnect(IMAPStore.java:726)
        at javax.mail.Service.connect(Service.java:364)
        at javax.mail.Service.connect(Service.java:245)
        at main.mail.ServiceIO.<init>(ServiceIO.java:27)
        at main.Main.main(Main.java:23)
Caused by: javax.net.ssl.SSLHandshakeException: sun.security.validator.Validator
Exception: PKIX path building failed: sun.security.provider.certpath.SunCertPath
BuilderException: 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:1949)
        at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:302)
        at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:296)
        at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.
java:1509)
        at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.jav
a:216)
        at sun.security.ssl.Handshaker.processLoop(Handshaker.java:979)
        at sun.security.ssl.Handshaker.process_record(Handshaker.java:914)
        at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1062)
        at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.
java:1375)
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1403
)
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1387
)
        at com.sun.mail.util.SocketFetcher.configureSSLSocket(SocketFetcher.java
:543)
        at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:348)
        at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:236)
        at com.sun.mail.iap.Protocol.<init>(Protocol.java:117)
        at com.sun.mail.imap.protocol.IMAPProtocol.<init>(IMAPProtocol.java:121)


        at com.sun.mail.imap.IMAPStore.newIMAPProtocol(IMAPStore.java:746)
        at com.sun.mail.imap.IMAPStore.protocolConnect(IMAPStore.java:689)
        ... 4 more
Caused by: sun.security.validator.ValidatorException: PKIX path building failed:
 sun.security.provider.certpath.SunCertPathBuilderException: unable to find vali
d certification path to requested target
        at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:387)
        at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.jav
a:292)
        at sun.security.validator.Validator.validate(Validator.java:260)
        at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.j
ava:324)
        at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerIm
pl.java:229)
        at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustMan
agerImpl.java:124)
        at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.
java:1491)
        ... 18 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to
 find valid certification path to requested target
        at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBu
ilder.java:146)
        at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCert
PathBuilder.java:131)
        at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280)
        at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:382)
        ... 24 more
Press any key to continue . . .

 

Apparently you have selected substituted SSL connection on which you want to X509 certificate

Maybe there is some one who could help getting and installing that certain certificate mentioned above?