If you a) use Connection Security=StartTLS or SSL/TLS in your Mail Server settings
b) use AVG Free (and probably other antivirus products) configured to check outgoing email (the default for AVG)

then you will get errors like:

24 Sep 2017 01:44:32,320 ERROR DialogErrorHandler,http-bio-8080-exec-8:69 - admin - Failed to send email to "Zz-Test,Zeb" <zzzzzzzz[at]gmail[dot]com>: Mail server connection failed; nested exception is javax.mail.MessagingException: Could not connect to SMTP host: mail.bigpond.com, port: 587;
  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. Failed messages: javax.mail.MessagingException: Could not connect to SMTP host: mail.bigpond.com, port: 587;
  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

The possible fixes are:

1. turn off checking of outgoing mail in the AV product
2. install the AV product's certificate in the Java certificate store

I did the first - assessing the second as complicated and needing work whenever Java is updated.

Regards, Tim G