SMTP Connection Pooling with Spring

EmailService.java

import javax.mail.internet.MimeMessage;

import org.nlab.smtp.pool.SmtpConnectionPool;
import org.nlab.smtp.transport.connection.ClosableSmtpConnection;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class EmailService {
  @Autowired
  private SmtpConnectionPool smtpConnectionPool;

  public void sendEmail() {
    try (ClosableSmtpConnection transport = smtpConnectionPool.borrowObject()) {
      MimeMessage mimeMessage = new MimeMessage(transport.getSession());

      ...

      transport.sendMessage(mimeMessage);
    }
  }
}

SMTPConfiguration.java

import java.util.Properties;

import javax.mail.Session;

import org.nlab.smtp.pool.SmtpConnectionPool;
import org.nlab.smtp.transport.factory.SmtpConnectionFactory;
import org.nlab.smtp.transport.factory.SmtpConnectionFactoryBuilder;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class SMTPConfiguration {
  @Bean
  public SmtpConnectionFactory smtpConnectionFactory() {
    return SmtpConnectionFactoryBuilder.newSmtpBuilder()
                .session(smtpSession())
                .protocol(...) 
                .host(...)
                .port(...)
                .username(...)
                .password(...)
      .build();
  }

  @Bean
  public SmtpConnectionPool smtpConnectionPool() {
    return new SmtpConnectionPool(SmtpConnectionFactoryBuilder.newSmtpBuilder().build());
  }

  @Bean
  public Properties smtpProperties() {
    Properties properties = new Properties();

    properties..put("mail.smtp.host", ...);
    properties..put("mail.smtp.port", ...);

    return properties;
  }

  @Bean
  public Session smtpSession() {
    return Session.getDefaultInstance(smtpProperties(), null);
  }
}

I'm facing the problem while integrate with Spring boot application. Please help me to resolve the problem.

https://stackoverflow.com/questions/63722608/spring-boot-with-smtp-connection-pool