Golang smtp.SendMail阻塞问题：深度解析与TLS连接解决方案

package main

import (
    "fmt"
    "net/smtp"
)

func main() {
    to := []string{"recipient@example.com"} // 替换为实际收件人邮箱
    from := "sender@example.com"             // 替换为实际发件人邮箱
    password := "your_password"              // 替换为实际邮箱密码
    smtpServer := "smtp.web.de:25"           // 替换为实际SMTP服务器地址和端口

    // 使用CRAMMD5Auth进行认证，也可以使用smtp.PlainAuth
    auth := smtp.CRAMMD5Auth(from, password)

    msg := []byte("Subject: Go Email Test\r\n\r\nHi, this is a test email from Go.")

    fmt.Println("Attempting to send email...")
    err := smtp.SendMail(smtpServer, auth, from, to, msg)
    if err != nil {
        fmt.Printf("Error sending email: %v\n", err)
    } else {
        fmt.Println("Email sent successfully!")
    }
}

func SendMail(addr string, a Auth, from string, to []string, msg []byte) error {
    // ... (连接建立和认证前的其他逻辑) ...

    // 检查服务器是否支持STARTTLS扩展
    if ok, _ := c.Extension("STARTTLS"); ok {
        config := &tls.Config{ServerName: c.serverName}
        // ... (可选的测试钩子) ...
        // 如果服务器支持，尝试升级到TLS连接
        if err = c.StartTLS(config); err != nil {
            return err // 如果StartTLS失败，返回错误
        }
    }
    // ... (认证和邮件发送逻辑) ...
}

package main

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

// SendMailTLS connects to the server at addr, default use TLS
func SendMailTLS(addr string, auth smtp.Auth, from string, to []string, msg []byte) error {
    host := addr
    if i := strings.LastIndex(addr, ":"); i > -1 {
        host = addr[:i]
    }

    // 配置TLS连接
    tlsconfig := &tls.Config{
        InsecureSkipVerify: false, // 生产环境中应为 false，确保验证服务器证书
        ServerName:         host,   // 必须设置，用于SNI和证书验证
    }

    // 使用tls.Dial直接建立TLS连接
    conn, err := tls.Dial("tcp", addr, tlsconfig)
    if err != nil {
        return fmt.Errorf("TLS Dial failed: %w", err)
    }
    defer conn.Close() // 确保连接在使用后关闭

    // 基于TLS连接创建SMTP客户端
    c, err := smtp.NewClient(conn, host)
    if err != nil {
        return fmt.Errorf("New SMTP client failed: %w", err)
    }
    defer c.Close() // 确保SMTP客户端在使用后关闭

    // 认证
    if auth != nil {
        if ok, _ := c.Extension("AUTH"); ok {
            if err = c.Auth(auth); err != nil {
                return fmt.Errorf("SMTP authentication failed: %w", err)
            }
        }
    }

    // 设置发件人
    if err = c.Mail(from); err != nil {
        return fmt.Errorf("Set sender failed: %w", err)
    }

    // 设置收件人
    for _, addr := range to {
        if err = c.Rcpt(addr); err != nil {
            return fmt.Errorf("Set recipient failed: %w", err)
        }
    }

    // 获取邮件数据写入器
    w, err := c.Data()
    if err != nil {
        return fmt.Errorf("Get data writer failed: %w", err)
    }

    // 写入邮件内容
    _, err = w.Write(msg)
    if err != nil {
        return fmt.Errorf("Write email data failed: %w", err)
    }
    err = w.Close()
    if err != nil {
        return fmt.Errorf("Close data writer failed: %w", err)
    }

    // 退出SMTP会话
    return c.Quit()
}

func main() {
    to := []string{"recipient@example.com"}
    from := "sender@example.com"
    password := "your_password"
    // 注意：对于直接TLS连接，通常使用465端口（SMTPS）
    // 或者某些服务器的587端口也可能直接支持TLS
    smtpServerAddr := "smtp.web.de:465" // 替换为实际SMTP服务器地址和端口，通常是465或587

    auth := smtp.PlainAuth("", from, password, strings.Split(smtpServerAddr, ":")[0]) // PlainAuth更常用

    msg := []byte("Subject: Go Email Test (TLS)\r\n\r\nHi, this is a test email from Go using TLS.")

    fmt.Println("Attempting to send email via TLS...")
    err := SendMailTLS(smtpServerAddr, auth, from, to, msg)
    if err != nil {
        fmt.Printf("Error sending email via TLS: %v\n", err)
    } else {
        fmt.Println("Email sent successfully via TLS!")
    }
}