JavaMail SMTP凭证验证，无需实际发送电子邮件

Question

有没有办法在不发送电子邮件或连接到POP/IMAP的情况下检查用户SMTP服务器凭证。 我试图编写的一些代码在它上面失败。 你能找到那里缺少的东西吗？

不要担心电子邮件/密码。我知道它在那里。

注意：如果您尝试了代码。提供正确凭证时应通过案例1。 如果失败，则有人更改密码。你应该使用其他一些电子邮件地址。

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 = "[email protected]"; 
    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

由于SMTP协议本身，您有此问题。协议中不强制认证。当你调用方法“connect”时，你至少会做一个“EHLO”命令来返回受支持的扩展。身份验证是在RFC中定义的扩展。如果服务器说它支持，并且您提供了凭据，则JavaMail将尝试执行“AUTH”命令而不是。事情是验证扩展信息可能只有在你完成了“STARTTLS”（也是一个扩展）之后才会发送，因为在一个空闲信道上使用例如PLAIN验证是不安全的。这就是为什么JavaMail在“STARTTLS”之后执行第二个“EHLO”以更新受支持的扩展。

因此，针对您的一个简单解决方案问题是始终将“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（否则不会），那么将发送“STARTTLS”命令，那么您将获得更新的扩展，并且如果需要，JavaMail可以执行“AUTH”。