SubEthaSMTP Unexpected error in the SMTP handler thread NoClassDefFoundError jakarta/mail/internet/AddressException

SubEthaSMTP server running fine on port 25001. After a successful authentication from my client code to Subethasmtp server i am having this error message org.subethamail.smtp.server.Session: Unexpected error in the SMTP handler thread java.lang.NoClassDefFoundError: jakarta/mail/internet/AddressException

This is my SMTPServerConfig class

 @Configuration
 public class SMTPServerConfig {

    private final SMTPServer smtpServer;
    private final SimpleMessageListener marketingMsgListener;
    private final UsernamePasswordValidator authValidator;
    private final EasyAuthenticationHandlerFactory easyAuth;

    public SMTPServerConfig(SimpleMessageListener marketingMsgListener) {
       authValidator = new SimpleAuthValidatorImpl();
       easyAuth = new EasyAuthenticationHandlerFactory(authValidator);
       this.marketingMsgListener = marketingMsgListener;

       this.smtpServer = SMTPServer
            .port(25001)
            .simpleMessageListener(this.marketingMsgListener)
            .requireAuth(true)
            .authenticationHandlerFactory(easyAuth)
            .hostName("localhost")
            .hideTLS(false)
            .enableTLS(true)
            .requireTLS(false)
            .requireAuth(true)
            .connectionTimeout(1, TimeUnit.MINUTES)
            .maxMessageSize(10000)
            .maxConnections(20)
            .build();

       this.smtpServer.stop();
       this.smtpServer.start();
   }

I am using usernamepasswordvalidator

 @Configuration
 public class SimpleAuthValidatorImpl implements UsernamePasswordValidator {

   private final String CREDENTIALS_LOGIN = "christclau";
   private final String CREDENTIALS_PASSWORD = "password";

   @Override
   public void login(String username, String password, MessageContext context) throws LoginFailedException {
      if(CREDENTIALS_LOGIN.equals(username) && CREDENTIALS_PASSWORD.equals(password)){
        System.out.println("yes Authenticated successfully");
      }else{
        System.err.println("no Invalid authentication !");
        throw new LoginFailedException();
      }
   }
 }

This is my mail client to send message to the server

 public void sendMail(Mail mail) {
    final String username = "christclau";
    final String password = "password";

    Properties prop = new Properties();
    prop.put("mail.smtp.host", "localhost");
    prop.put("mail.smtp.port", "25001");
    prop.put("mail.smtp.auth", "true");
    //prop.put("mail.debug", "true");
    //prop.put("mail.smtp.starttls.enable", "true"); //TLS
    
    Session session = Session.getInstance(prop,
        new javax.mail.Authenticator() {
            @Override
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });
    session.setDebug(true);

    try {

        Message message = new MimeMessage(session);
        message.setFrom(new InternetAddress(mail.getFrom()));
        message.setRecipients(
                Message.RecipientType.TO,
                InternetAddress.parse(mail.getTo())
        );
        message.setSubject(mail.getSubject());
        message.setText("HI you have done sending mail with outlook");

        Transport.send(message);

        System.out.println("Done");

    } catch (MessagingException e) {
        e.printStackTrace();
    }
  }

This is the output message

 DEBUG: setDebug: Jakarta Mail version 1.6.7
 DEBUG: getProvider() returningjavax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Oracle]
 DEBUG SMTP: need username and password for authentication
 DEBUG SMTP: protocolConnect returning false, host=localhost, user=MY PC, password=<null>
 DEBUG SMTP: useEhlo true, useAuth true
 DEBUG SMTP: trying to connect to host "localhost", port 25001, isSSL false 220 localhost ESMTP SubEthaSMTP null
 DEBUG SMTP: connected to host "localhost", port: 25001
 EHLO DESKTOP-FFJA5IP
 250-localhost
 250-8BITMIME
 250-SIZE 10000
 250-STARTTLS
 250-CHUNKING
 250-AUTH PLAIN LOGIN
 250 Ok
 DEBUG SMTP: Found extension "8BITMIME", arg ""
 DEBUG SMTP: Found extension "SIZE", arg "10000"
 DEBUG SMTP: Found extension "STARTTLS", arg ""
 DEBUG SMTP: Found extension "CHUNKING", arg ""
 DEBUG SMTP: Found extension "AUTH", arg "PLAIN LOGIN"
 DEBUG SMTP: Found extension "Ok", arg ""
 DEBUG SMTP: protocolConnect login, host=localhost, user=christclau, password=<non-null>
 DEBUG SMTP: Attempt to authenticate using mechanisms: LOGIN PLAIN DIGEST-MD5 NTLM XOAUTH2
 DEBUG SMTP: Using mechanism LOGIN
 DEBUG SMTP: AUTH LOGIN command trace suppressed
 yes Authenticated successfully
 DEBUG SMTP: AUTH LOGIN succeeded
 DEBUG SMTP: use8bit false
 MAIL FROM:<[e-mail]>
 2022-02-26 11:21:40.081 ERROR 9452 --- [127.0.0.1:50466] org.subethamail.smtp.server.Session      : Unexpected error in the SMTP handler thread

 java.lang.NoClassDefFoundError: jakarta/mail/internet/AddressException
 at org.subethamail.smtp.internal.command.MailCommand.execute(MailCommand.java:74) ~[subethasmtp-6.0.1.jar:na]
at org.subethamail.smtp.internal.server.RequireTLSCommandWrapper.execute(RequireTLSCommandWrapper.java:32) ~[subethasmtp-6.0.1.jar:na]
at org.subethamail.smtp.internal.server.RequireAuthCommandWrapper.execute(RequireAuthCommandWrapper.java:35) ~[subethasmtp-6.0.1.jar:na]
at org.subethamail.smtp.internal.server.CommandHandler.handleCommand(CommandHandler.java:86) ~[subethasmtp-6.0.1.jar:na]
at org.subethamail.smtp.server.Session.runCommandLoop(Session.java:261) ~[subethasmtp-6.0.1.jar:na]
at org.subethamail.smtp.server.Session.run(Session.java:170) ~[subethasmtp-6.0.1.jar:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]
 Caused by: java.lang.ClassNotFoundException: jakarta.mail.internet.AddressException
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) ~[na:na]
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) ~[na:na]
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[na:na]
... 9 common frames omitted

This is my dependency

 <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-mail</artifactId>
 </dependency>
 <dependency>
        <groupId>com.github.davidmoten</groupId>
        <artifactId>subethasmtp</artifactId>
        <version>6.0.1</version>
 </dependency>
 

Answer

Spring Boot 2.6 and earlier still use the javax.mail namespace for JakartaMail (formerly JavaMail), while your SubEtha SMTP version seems to expect the jakarta.mail namespace. Spring Boot explicitly specifies versions of the JakartaMail package, see Spring Boot Dependency Versions and search for jakarta.mail. For Spring Boot 2.6.4, it lists version 1.6.7, and that version is still based on the javax.mail namespace.

You need to downgrade SubEtha SMTP to a version that still uses the javax.mail namespace of JakartaMail/JavaMail, and wait for Spring Boot 3 before using a version that needs the jakarta.mail namespace.

In theory, you can also set the Maven property jakarta-mail.version to 2.0.1, but this could cause problems in other parts of Spring Boot, so I do not recommend that.