Send mail gives EmailSendException

Hello everyone.
I'm new to Appian. My problem happens when I try to run a process model containing only a "send mail" task. It's configured to send mail to only one recipient.
I've configured custom.properties as described into the documentation but despite I am using plain SMTP (and not SMTPS) on port 25, into the application-server.log is reported:

ERROR com.appiancorp.ap2.mail.Mail - Could not send mail via 192.168.1.252:25 to <mail@address.com>
[Appian Work Item - 182 - ProcessExec00 : UnattendedJavaActivityRequest] ERROR com.appiancorp.process.runtime.activities.SendEmailActivity - 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
javax....

OriginalPostID-243175