Hey there, let's break down why your email send is failing and fix it step by step.

Core Issue #

SMTP port 25 is not designed for direct SSL connections—it's meant for plaintext connections that can be upgraded to encryption using STARTTLS. For direct SSL-based SMTP, the standard port is 465 (often called SMTPS). Your current configuration forces SSL on port 25, which most mail servers don't support, leading to the failure. Also, there's a tiny syntax error in your socket factory class line (extra closing quote) that might be compounding issues.

Fix Options #

Option 1: Switch to Port 465 (Direct SSL) #

If your email provider supports port 465 (most major providers do), adjust your properties like this:

Properties properties = System.getProperties();
properties.setProperty("mail.smtp.host", host);
properties.put("mail.smtp.port", "465");
properties.put("mail.smtp.ssl.enable", "true");
properties.put("mail.smtp.auth", "true");
// Modern JavaMail versions might not need these socket factory settings, but they're safe to keep
properties.put("mail.smtp.socketFactory.fallback", "false");
properties.put("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory"); // Fixed the quote issue

Authenticator auth = new Authenticator() {
    @Override
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication("your-email@domain.com", "your-password");
    }
};

Option 2: Use STARTTLS on Port 25 #

If you must use port 25 (e.g., due to network restrictions), use STARTTLS to upgrade the plaintext connection to encrypted. Replace your SSL-related properties with STARTTLS settings:

Properties properties = System.getProperties();
properties.setProperty("mail.smtp.host", host);
properties.put("mail.smtp.port", "25");
properties.put("mail.smtp.starttls.enable", "true"); // Enable STARTTLS instead of direct SSL
properties.put("mail.smtp.starttls.required", "true"); // Force encryption, fail if upgrade isn't possible
properties.put("mail.smtp.auth", "true");

Authenticator auth = new Authenticator() {
    @Override
    protected PasswordAuthentication getPasswordAuthentication() {
        return new PasswordAuthentication("your-email@domain.com", "your-password");
    }
};

Note: For this to work, your mail server must support STARTTLS on port 25.

Additional Checks #

• Verify that your email provider allows SMTP access from your network (some block ports 25/465 for non-trusted IPs).
• Double-check your username and password—typos here are an extremely common culprit.
• Ensure you're using a recent version of javax.mail (or jakarta.mail, its modern successor) to avoid outdated SSL compatibility bugs.

内容的提问来源于stack exchange，提问作者Vinayak CD