Use the following command in importing a pkcs12 keystore to Java keystore:

The keytool is normally found in $JAVA_HOME/jre/bin (i.e. the $JAVA_HOME variable is where you've installed JDK).

keytool -importkeystore -v -srckeystore <SRC_KEYSTORE_FILE> -srcstoretype pkcs12 -destkeystore <DEST_KEYSTORE_FILE> -deststoretype JKS -storepass <KEYSTORE_PASSWORD>

Gradle dependency

compile group: 'com.sun.mail', name: 'javax.mail', version: '1.6.2'

Import to the Class File

import javax.mail.*;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import java.util.Properties;

The Template

// Recipient's email ID needs to be mentioned.
String to = <RECIPIENTS_EMAIL>;

//Sender's email ID needs to be mentioned
String from = <SENDERS_EMAIL>;

//The subject of the email.
String subject = <EMAIL_SUBJECT>;

//The body of the email.
String body = <EMAIL_BODY>;

//SMTP Server
final String host = <SMTP_SERVER>;

//SMTP Port
final String port = <SMTP_PORT>;

//SMTP Username
final String username = <USERNAME>;

//SMTP Password
final String password = <PASSWORD>;

try {
    //SMTP Configuration
    Properties props = new Properties();
    props.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
    props.put("mail.smtp.auth", "true");
    props.put("mail.smtp.starttls.enable", "true");
    props.put("mail.smtp.host", host);
    props.put("mail.smtp.port", port);

    // Create a session object.
    Session session = Session.getInstance(props,
        new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        }
    );

    // Create a message object.
    Message message = new MimeMessage(session);

    // Set From: header field of the header.
    message.setFrom(new InternetAddress(from));

    // Set To: header field of the header.
    message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));

    // Set Subject: header field
    message.setSubject(subject);

    // Now set the actual message
    message.setText(body);

    // Send message
    Transport.send(message);

    System.out.println("Sent message successfully....");

} catch (MessagingException e) {
    throw new RuntimeException(e);
}