Using SMTP with STARTTLS

Using SMTP with STARTTLS

Hello,
My customer is asking me to use SMTP with STARTTLS activated but when i do, i have the following error :

Primary SMTP server : Connection Failed. 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

I believe it's due to the fact that they have their own Certificate Authority so i need to import root certificates in a truststore or a keystore but i don't know where it is located.

Could you please advise?

The build number is 14420.

Thank you in advance
Philippe