Agreed: This seems to be more related to the configuration of the Outbound email server (relaying?) than with PRPC itself.

What you could do to diagnose this further -  Download Mozilla Thunderbird Email Client and hook it into the same SMTP server as you are using with PRPC - and see whether you receive a similar problem or not.

If you do receive the same problem , then it probably is down to the setup of the SMTP server; if Thunderbird works: then you perhaps want to try a simple standalone Java Program that uses the 'Javamail' JAR to do the same test; the following Java Code can be used a starting point: (change the 'mResponseHostName' etc for your email coordinates/authentication of course).

You will need to download (the exact version) of Javamail that your PRPC system is using here and use the same (exact) version of the JVM for a 'fair-test' here.

Actually: I just realized that this does a 'test connection' only - and you have confirmed that is already working.

You will need to add in a couple of lines to this code below to make it actually send a message - this (and many other) example can be used to do this.

package com.pega.gcs.emailer;

import java.util.Properties;
import javax.mail.Transport;
import javax.mail.Session;
import javax.mail.MessagingException;

public class TestSMTP {

    private static final String SSL_FACTORY = "javax.net.ssl.SSLSocketFactory";

    private String mResponseHostName;
    private String mResponsePort = null;
    private String mConnectionTimeout;
    private String mResponseUserID = "";
    private String mResponsePassword = "";
    private boolean mResponseUseSSL;

    java.io.OutputStream output = new java.io.OutputStream() {
        private StringBuilder string = new StringBuilder();

        public void write(int b) throws java.io.IOException {
            this.string.append((char) b);
        }

        public String toString() {
            return this.string.toString();
        }
    };

    public static void main(String[] args) {
        TestSMTP me = new TestSMTP();
        me.run();

    }

    public void run() {
        java.io.PrintStream ps = new java.io.PrintStream(output);
        // See : http://stackoverflow.com/questions/14064111/java-mail-mystery-smtp-blocked
        java.util.Properties sysProperties = System.getProperties();
        sysProperties.put("java.net.preferIPv4Stack", "true");
      

        mResponseHostName = "mailhost;
        mResponsePort = "25";
        mConnectionTimeout = "500";
        mResponseUseSSL = false;
        mResponseUserID = "user@mailhost";
        mResponsePassword = "password";


        try {
            System.out.println("Setting up SMTP details");
            Transport transport = null;
            try {
                Properties args = new Properties();
                args.put("mail.smtp.host", mResponseHostName);

                if (mResponsePort != null && mResponsePort.length() > 0) {
                    args.put("mail.smtp.port", mResponsePort);
                    args.put("mail.smtps.port", mResponsePort);
                }

                args.put("mail.transport.protocol", "smtp");
                args.put("mail.smtp.timeout", mConnectionTimeout);
                args.put("mail.smtp.connectiontimeout", mConnectionTimeout);

                if (mResponseUseSSL) {
                    args.put("mail.smtps.socketFactory.class", SSL_FACTORY);
                    args.put("mail.smtps.socketFactory.fallback", "false");
                }

                System.out.println("Making SMTP connect");
                Session session = Session.getInstance(args, null);
                session.setDebugOut(ps);
                session.setDebug(true);
              
                transport = mResponseUseSSL ? session.getTransport("smtps") : session.getTransport("smtp");
                transport.connect(mResponseHostName, mResponseUserID, mResponsePassword);
                System.out.println("Closing SMTP connection");
                transport.close();

            } finally {
                if (transport != null) {
                    transport.close();
                }
            }
        } catch (MessagingException e) {

        
            throw (new RuntimeException(e));

        }
        finally {  System.out.println(output.toString()); }

    }
}