JavaMail SMTP 凭据验证，无需实际发送电子邮件

Question

有没有一种方法可以在不发送电子邮件或连接到 POP/IMAP 的情况下检查用户 SMTP 服务器凭据。我尝试编写的一些代码失败了。你能找到那里缺少的东西吗？

不用担心电子邮件/密码。我知道它在那里。

注意:如果您正在试用代码。当提供正确的凭据时，案例 1 应该通过。如果失败，则说明有人更改了密码。您应该使用其他电子邮件地址。

import java.util.Properties;

import javax.mail.Authenticator;
import javax.mail.MessagingException;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;

public class EmailTest {

 public static void main(String[] args) {
  EmailHelper eh = new EmailHelper();

  /* GMail Setting for SMTP using STARTTLS */
  String name = "AAA";
  String email = "mymasterpainter@gmail.com";
  String smtpHost = "smtp.gmail.com";
  String serverPort = "587";
  String requireAuth = "true";
  String dontuseAuth = "false";
  String userName = email; // same as username for GMAIL
  String password = "zaq12wsx";
  String incorrectPassword = "someRandomPassword";
  String enableSTARTTLS = "true";
  String dontenableSTARTTLS = "false";

  try {
   /* only valid case */
   eh.sendMail(name, email, smtpHost, serverPort, requireAuth,
     userName, password, enableSTARTTLS);
   System.out.println("Case 1 Passed");

   /* should fail since starttls is required for GMAIL. */
   eh.sendMail(name, email, smtpHost, serverPort, requireAuth,
     userName, password, dontenableSTARTTLS);
   System.out.println("Case 2 Passed");

   /* should fail since GMAIL requires authentication */
   eh.sendMail(name, email, smtpHost, serverPort, dontuseAuth, "", "",
     dontenableSTARTTLS);
   System.out.println("Case 3 Passed");

   /* should fail. password is incorrect and starttls is not enabled */
   eh.sendMail(name, email, smtpHost, serverPort, requireAuth,
     userName, incorrectPassword, dontenableSTARTTLS);
   System.out.println("Case 4 Passed");
  } catch (MessagingException e) {
   e.printStackTrace();
  }
 }

}

class EmailHelper {

 private Properties properties = null;
 private Authenticator authenticator = null;
 private Session session = null;

 public void sendMail(String name, String email, String smtpHost,
   String serverPort, String requireAuth, String userName,
   String password, String enableSTARTTLS) throws MessagingException {
  properties = System.getProperties();
  properties.put("mail.smtp.host", smtpHost);
  properties.put("mail.smtp.port", serverPort);
  properties.put("mail.smtp.starttls.enable", enableSTARTTLS);
  properties.put("mail.smtp.auth", requireAuth);
  properties.put("mail.smtp.timeout", 20000);

  authenticator = new SMTPAuthenticator(userName, password);

  session = Session.getInstance(properties, authenticator);

  // session.setDebug(true);

  Transport tr = session.getTransport("smtp");
  tr.connect();
  /*
   * do I need more than just connect? Since when i try to send email with
   * incorrect credentials it fails to do so. But I want to check
   * credentials without sending an email. Assume that POP3/IMAP username
   * is not same as the SMTP username, since that might be one of the
   * cases
   */
 }
}

class SMTPAuthenticator extends Authenticator {

 private String userName = null;
 private String password = null;

 public SMTPAuthenticator(String userName, String password) {
  this.userName = userName;
  this.password = password;

 }

 @Override
 public PasswordAuthentication getPasswordAuthentication() {
  return new PasswordAuthentication(userName, password);
 }
}

Answer 1

您遇到此问题是因为 SMTP 协议(protocol)本身。身份验证在协议(protocol)中不是强制性的。当您调用“connect”方法时，您至少会执行一个“EHLO”命令，该命令返回支持的扩展。身份验证是 RFC 中定义的扩展。 JavaMail 将尝试执行“AUTH”命令仅如果服务器说它支持并且您提供了凭据。事情是身份验证扩展信息只能在您完成“STARTTLS”(这也是一个扩展)之后发送，因为在清晰的 channel 上使用例如 PLAIN 身份验证是不安全的。这就是为什么 JavaMail 在“STARTTLS”之后执行第二个“EHLO”以更新支持的扩展。

因此，解决您问题的一个简单方法是始终将“mail.smtp.starttls.enable”设置为 true。在 JavaMail(至少 1.4.5)中，这意味着如果服务器说它支持(否则不支持)，将发送“STARTTLS”命令，然后您将获得更新的扩展名，如果需要，JavaMail 可能会执行“AUTH”。