Send email with Java (Jakarta Mail) over SMTP

<!-- Maven -->
<dependency>
  <groupId>com.sun.mail</groupId>
  <artifactId>jakarta.mail</artifactId>
  <version>2.0.1</version>
</dependency>

import jakarta.mail.*;
import jakarta.mail.internet.*;
import java.util.Properties;

public class SendMail {
  public static void main(String[] args) throws Exception {
    String host = System.getenv().getOrDefault("SMTP_HOST", "smtp.queensmtp.com");
    int port = Integer.parseInt(System.getenv().getOrDefault("SMTP_PORT", "587"));
    String user = System.getenv("SMTP_USER"), pass = System.getenv("SMTP_PASS");

    Properties p = new Properties();
    p.put("mail.smtp.host", host);
    p.put("mail.smtp.port", String.valueOf(port));
    p.put("mail.smtp.auth", "true");
    if (port == 465) {
      p.put("mail.smtp.ssl.enable", "true");          // implicit TLS
    } else {
      p.put("mail.smtp.starttls.enable", "true");     // STARTTLS on 587
      p.put("mail.smtp.starttls.required", "true");
    }
    p.put("mail.smtp.connectiontimeout", "20000");
    p.put("mail.smtp.timeout", "20000");

    Session session = Session.getInstance(p, new Authenticator() {
      protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication(user, pass);
      }
    });

    MimeMessage msg = new MimeMessage(session);
    msg.setFrom(new InternetAddress("hello@yourdomain.com", "Your App"));
    msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse("customer@example.com"));
    msg.setSubject("Your receipt");
    msg.setContent("<h1>Thanks for your order</h1><p>#1042 is confirmed.</p>", "text/html; charset=utf-8");
    Transport.send(msg);
    System.out.println("sent");
  }
}

// Spring Boot: application.properties
// spring.mail.host=smtp.queensmtp.com
// spring.mail.port=587
// spring.mail.username=${SMTP_USER}
// spring.mail.password=${SMTP_PASS}
// spring.mail.properties.mail.smtp.auth=true
// spring.mail.properties.mail.smtp.starttls.enable=true