Hey there, that 535 "Invalid user or password" error when sending emails via Yandex SMTP is a pretty common pitfall—let’s walk through the most likely fixes to get your app working:

First, Double-Check Your Credentials #

• Username must be your full Yandex email address: Don’t just use yourname—it needs to be yourname@yandex.com (or your custom domain if you’ve set one up).
• Password considerations:
  • If you have two-factor authentication (2FA) enabled on your Yandex account, your regular login password won’t work here. You need to generate a special app password for your JavaMail application. Head to your Yandex account’s Security settings, find "App passwords", create a new one for mail, and use that password instead.
  • Make sure you’re not accidentally using a wrong password (typos happen—double-check case sensitivity too!).

Verify SMTP Server Configuration #

Yandex’s SMTP setup has specific requirements—ensure your Properties are configured correctly:

• Correct host: smtp.yandex.com
• Valid ports: Use 465 with SSL or 587 with TLS (don’t mix these up)
• Enable authentication and encryption flags

Here’s a corrected configuration snippet to replace your current props setup:

Properties props = System.getProperties();
// For SSL (port 465)
props.put("mail.smtp.host", "smtp.yandex.com");
props.put("mail.smtp.port", "465");
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.ssl.enable", "true");

// OR for TLS (port 587)
// props.put("mail.smtp.host", "smtp.yandex.com");
// props.put("mail.smtp.port", "587");
// props.put("mail.smtp.auth", "true");
// props.put("mail.smtp.starttls.enable", "true");

Ensure Your Account Allows SMTP Access #

Log into your Yandex Mail account, go to Settings > Mail clients, and make sure the "Allow access to mail from external applications" option is turned on. Yandex disables this by default for some accounts, so this is a critical step.

Fix the Session Authentication #

Make sure you’re properly passing the credentials to the Session object using an Authenticator:

Session session = Session.getInstance(props, new javax.mail.Authenticator() {
    @Override
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication(from, pass);
    }
});

Test Again #

After making these changes, try sending the email again. If you still run into issues, check if your network is blocking SMTP traffic (some firewalls or ISPs restrict port 465/587), but the above steps should resolve 90% of authentication failures with Yandex.

内容的提问来源于stack exchange，提问作者Mr. Kevin