Could not convert socket to TLS [EmailSender] WARNING

Hi,

Could anyone tell what does it mean those errors in Server.log?

"Could not convert socket to TLS WARNING [EmailSender]"

How to avoid it?

2017-09-03 06:41:47 WARNING [StringTable] A string could not be found in server side StringTable. Suppressing this warning for another 60000 ms.
2017-09-03 06:41:57 WARNING [EmailSender] Delaying 1000ms before starting retry 1 of 1, error: org.apache.commons.mail.EmailException: Sending the email to the following server failed : email.domain.net:25
2017-09-03 06:41:58 WARNING [EmailSender] Sending email caused an exception: Sending the email to the following server failed : email.domain.net:25

Could not convert socket to TLS
sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
unable to find valid certification path to requested target: com.dynatrace.diagnostics.server.shared.email.EmailSender a:207
org.apache.commons.mail.EmailException: Sending the email to the following server failed : email.domain.net:25
at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1421)
at org.apache.commons.mail.Email.send(Email.java:1448)
at com.dynatrace.diagnostics.server.shared.email.ImageHtmlEmail.resend(SourceFile:106)
at com.dynatrace.diagnostics.server.shared.email.EmailSender.a(SourceFile:322)
at com.dynatrace.diagnostics.server.shared.email.EmailSender.sendEmailSub(SourceFile:163)
at com.dynatrace.diagnostics.server.shared.email.EmailSender.sendEmailSub(SourceFile:110)
at com.dynatrace.diagnostics.server.ServerAccessController.sendEmail(SourceFile:5178)
at com.dynatrace.diagnostics.core.incidents.IncidentMailSender.a(SourceFile:489)
at com.dynatrace.diagnostics.core.incidents.IncidentMailSender.a(SourceFile:516)
at com.dynatrace.diagnostics.core.incidents.IncidentMailSender.a(SourceFile:410)
at com.dynatrace.diagnostics.core.incidents.IncidentMailSender.a(SourceFile:65)
at com.dynatrace.diagnostics.core.incidents.IncidentMailSender$BulkIncidentMailTask.run(SourceFile:525)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)
Caused by: javax.mail.MessagingException: Could not convert socket to TLS;
  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.startTLS(SMTPTransport.java:1907)
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:666)
at javax.mail.Service.connect(Service.java:295)
at javax.mail.Service.connect(Service.java:176)
at javax.mail.Service.connect(Service.java:125)
at javax.mail.Transport.send0(Transport.java:194)
at javax.mail.Transport.send(Transport.java:124)
at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1411)
... 13 more
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: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.java: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:549)
at com.sun.mail.util.SocketFetcher.startTLS(SocketFetcher.java:486)
at com.sun.mail.smtp.SMTPTransport.startTLS(SMTPTransport.java:1902)
... 20 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:387)
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:324)
at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:229)
at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:124)
at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1491)
... 30 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:141)
at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:126)
at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280)
at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:382)
... 36 more