Java Mail调用Gmail SMTP 587端口发送邮件时出现连接超时问题

Hey there, I’ve dealt with this exact timeout issue a few times when working with Gmail’s SMTP server—let’s walk through the most likely fixes step by step:

1. First, Rule Out Network/Firewall Issues #

The "Operation timed out" error usually means your machine can’t reach smtp.gmail.com:587 at all. Let’s verify that first:

• Run a quick connectivity test using telnet or nc (netcat) in your terminal:

  telnet smtp.gmail.com 587
  # Or if telnet isn't installed:
  nc -zv smtp.gmail.com 587
  

• If the test fails, check:
  • Your local firewall (Windows Defender Firewall, macOS Firewall, etc.)—make sure it’s not blocking outbound connections on port 587.
  • Your company/school network—many organizations block SMTP ports to prevent spam. You might need to ask your network admin for exceptions, or try port 465 (SSL) instead.
  • Your ISP—some ISPs restrict SMTP ports; switching to port 465 could work here too.

2. Fix Gmail’s Security Settings (Critical!) #

Gmail no longer allows regular passwords for third-party apps like Java Mail unless you use an App Password:

• First, enable 2-Step Verification on your Google Account (you can find this under "Security" in your Google Account settings).
• Once 2-Step Verification is on, go to the "App Passwords" section (still under Security). Generate a 16-character app password specifically for your Java Mail program.
• Replace the PASSWORD value in your code with this app password—your regular Gmail password won’t work here anymore.

3. Make Sure Your Java Code Has the Required SMTP Properties #

Looking at your code snippet, you’re missing two critical properties that Gmail’s SMTP server requires:

// Add these to your properties setup
properties.put("mail.smtp.auth", "true"); // Enables SMTP authentication
properties.put("mail.smtp.starttls.enable", "true"); // Required for port 587 (STARTTLS encryption)

• If you switch to port 465 (SSL), replace the STARTTLS line with:

  properties.put("mail.smtp.ssl.enable", "true");
  

• Also, it’s safer to use an integer for the port instead of a string—change String PORT = "587"; to int PORT = 587; and update the property to use the integer value.

4. Check for Proxy Issues #

If you’re behind a corporate proxy, you’ll need to add proxy settings to your properties:

properties.put("mail.smtp.proxy.host", "your-proxy-server");
properties.put("mail.smtp.proxy.port", "your-proxy-port");
// If your proxy requires authentication:
properties.put("mail.smtp.proxy.user", "proxy-username");
properties.put("mail.smtp.proxy.password", "proxy-password");

Test with a Minimal Working Code #

Here’s a complete, stripped-down example you can use to test if the connection works:

import javax.mail.*;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import java.util.Properties;

public class GmailTest {
    public static void main(String[] args) {
        String host = "smtp.gmail.com";
        int port = 587;
        String username = "myname@gmail.com";
        String appPassword = "your-generated-app-password"; // Use your App Password here

        Properties props = new Properties();
        props.put("mail.smtp.host", host);
        props.put("mail.smtp.port", port);
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.starttls.enable", "true");

        // Create session with authentication
        Session session = Session.getInstance(props, new Authenticator() {
            @Override
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, appPassword);
            }
        });

        try {
            Message msg = new MimeMessage(session);
            msg.setFrom(new InternetAddress(username));
            msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse("your-test-email@example.com"));
            msg.setSubject("SMTP Connection Test");
            msg.setText("If you get this, the connection works!");

            Transport.send(msg);
            System.out.println("Mail sent successfully!");
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}

Start with the network test first—if that’s failing, none of the code fixes will help. Once connectivity is confirmed, work through the security settings and code adjustments.

内容的提问来源于stack exchange，提问作者roger_that