Golang smtp.SendMail阻塞问题深度解析与TLS解决方案

func SendMail(addr string, a Auth, from string, to []string, msg []byte) error {
    // ...
    // 尝试建立普通TCP连接
    // ...

    // 检查服务器是否支持STARTTLS
    if ok, _ := c.Extension("STARTTLS"); ok {
        config := &tls.Config{ServerName: c.serverName}
        // ...
        if err = c.StartTLS(config); err != nil {
            return err // 升级失败
        }
    }
    // ...
    // 继续发送邮件
    // ...
}

package main

import (
    "crypto/tls"
    "fmt"
    "net/smtp"
    "strings"
)

// SendMailTLS connects to the server at addr, default use TLS
// This function establishes an explicit TLS connection from the start.
func SendMailTLS(addr string, auth smtp.Auth, from string, to []string, msg []byte) error {
    host := strings.Split(addr, ":")[0] // Extract host from addr (e.g., "smtp.web.de")

    // Configure TLS connection.
    // ServerName must match the hostname the TLS certificate is issued for.
    tlsconfig := &tls.Config{
        ServerName: host,
        // InsecureSkipVerify: false, // Default to false for security in production
        // For self-signed certificates or testing environments where you want to skip certificate validation,
        // you might set InsecureSkipVerify to true. However, this is NOT recommended for production.
        // InsecureSkipVerify: true, // Use with caution!
    }

    // Establish a TLS connection directly
    conn, err := tls.Dial("tcp", addr, tlsconfig)
    if err != nil {
        return fmt.Errorf("failed to dial TLS server %s: %w", addr, err)
    }
    defer conn.Close() // Ensure connection is closed after use

    // Create a new SMTP client over the established TLS connection
    c, err := smtp.NewClient(conn, host)
    if err != nil {
        return fmt.Errorf("failed to create SMTP client for %s: %w", host, err)
    }
    defer c.Close() // Ensure client is closed after use

    // Authenticate if auth is provided
    if auth != nil {
        if ok, _ := c.Extension("AUTH"); ok {
            if err = c.Auth(auth); err != nil {
                return fmt.Errorf("SMTP authentication failed: %w", err)
            }
        } else {
            return fmt.Errorf("SMTP server does not support AUTH extension")
        }
    }

    // Set the sender (MAIL FROM command)
    if err = c.Mail(from); err != nil {
        return fmt.Errorf("failed to set sender %s: %w", from, err)
    }

    // Set the recipients (RCPT TO command)
    for _, recipient := range to {
        if err = c.Rcpt(recipient); err != nil {
            return fmt.Errorf("failed to set recipient %s: %w", recipient, err)
        }
    }

    // Get a writer for the email body (DATA command)
    w, err := c.Data()
    if err != nil {
        return fmt.Errorf("failed to get data writer: %w", err)
    }

    // Write the email body
    _, err = w.Write(msg)
    if err != nil {
        return fmt.Errorf("failed to write email body: %w", err)
    }

    // Close the writer to send the email
    err = w.Close()
    if err != nil {
        return fmt.Errorf("failed to close data writer: %w", err)
    }

    // Quit the SMTP session (QUIT command)
    return c.Quit()
}

func main() {
    // 替换为你的实际SMTP服务器详情
    // 示例使用web.de，通常端口465用于隐式TLS
    smtpServerAddr := "smtp.web.de:465"
    senderEmail := "your_email@web.de" // 你的发件邮箱
    senderPassword := "your_password"   // 你的邮箱密码
    recipientEmail := "recipient@example.com" // 收件人邮箱

    // 邮件内容，包括Subject和空行分隔符
    message := []byte("Subject: Go SMTP TLS Test\r\n" +
        "From: " + senderEmail + "\r\n" +
        "To: " + recipientEmail + "\r\n" +
        "\r\n" + // 邮件头和邮件体之间的空行
        "This is a test email sent via Go with explicit TLS connection.")

    // 根据SMTP服务器要求选择认证方式，CRAMMD5Auth或PlainAuth
    // 这里使用PlainAuth作为示例
    auth := smtp.PlainAuth("", senderEmail, senderPassword, strings.Split(smtpServerAddr, ":")[0])

    fmt.Printf("Attempting to send email via explicit TLS to %s...\n", smtpServerAddr)
    err := SendMailTLS(smtpServerAddr, auth, senderEmail, []string{recipientEmail}, message)
    if err != nil {
        fmt.Printf("Error sending email: %v\n", err)
    } else {
        fmt.Println("Email sent successfully!")
    }
}