深入解析SMTP协议：理解邮件传输的核心机制与角色分工

<?php
// 这是一个极简的SMTP监听器伪代码，仅用于概念说明
// 实际生产级SMTP服务器远比这复杂，需要处理完整的SMTP状态机、错误、安全等

$host = '0.0.0.0'; // 监听所有可用接口
$port = 25;        // SMTP默认端口

set_time_limit(0); // 永不超时
ob_implicit_flush(); // 实时输出

if (($socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP)) === false) {
    echo "socket_create() failed: reason: " . socket_strerror(socket_last_error()) . "\n";
    exit();
}

if (socket_bind($socket, $host, $port) === false) {
    echo "socket_bind() failed: reason: " . socket_strerror(socket_last_error($socket)) . "\n";
    exit();
}

if (socket_listen($socket, 5) === false) { // 允许5个连接排队
    echo "socket_listen() failed: reason: " . socket_strerror(socket_last_error($socket)) . "\n";
    exit();
}

echo "SMTP Server listening on $host:$port...\n";

while (true) {
    // 接受新的客户端连接
    if (($client = socket_accept($socket)) === false) {
        echo "socket_accept() failed: reason: " . socket_strerror(socket_last_error($socket)) . "\n";
        continue;
    }

    echo "Client connected.\n";
    socket_write($client, "220 example.com ESMTP Service Ready\r\n"); // 发送欢迎信息

    $mail_from = '';
    $rcpt_to = [];
    $data_mode = false;
    $mail_content = '';

    while (true) {
        $buffer = socket_read($client, 2048, PHP_NORMAL_READ); // 读取客户端数据
        if ($buffer === false || trim($buffer) == '') {
            break; // 连接关闭或无数据
        }

        $command = strtoupper(trim(substr($buffer, 0, strpos($buffer, ' '))));
        $args = trim(substr($buffer, strpos($buffer, ' ') + 1));

        if ($data_mode) {
            // 在DATA模式下，所有行都是邮件内容的一部分，直到遇到单独的'.'
            if (trim($buffer) == '.') {
                $data_mode = false;
                echo "Received Mail from: $mail_from to: " . implode(', ', $rcpt_to) . "\n";
                // 实际应用中，这里会将 $mail_content 存储到文件或数据库
                echo "Mail Content (truncated):\n" . substr($mail_content, 0, 200) . "...\n";
                socket_write($client, "250 Ok: queued as 12345\r\n");
                $mail_from = ''; $rcpt_to = []; $mail_content = '';
            } else {
                $mail_content .= $buffer;
            }
        } else {
            switch ($command) {
                case 'EHLO':
                case 'HELO':
                    socket_write($client, "250-example.com\r\n250-PIPELINING\r\n250-SIZE 10240000\r\n250-VRFY\r\n250-ETRN\r\n250-AUTH PLAIN LOGIN\r\n250-ENHANCEDSTATUSCODES\r\n250-8BITMIME\r\n250 DSN\r\n");
                    break;
                case 'MAIL':
                    if (preg_match('/FROM:<(.*)>/i', $args, $matches)) {
                        $mail_from = $matches[1];
                        socket_write($client, "250 2.1.0 Ok\r\n");
                    } else {
                        socket_write($client, "501 Syntax error in parameters or arguments\r\n");
                    }
                    break;
                case 'RCPT':
                    if (preg_match('/TO:<(.*)>/i', $args, $matches)) {
                        $rcpt_to[] = $matches[1];
                        socket_write($client, "250 2.1.5 Ok\r\n");
                    } else {
                        socket_write($client, "501 Syntax error in parameters or arguments\r\n");
                    }
                    break;
                case 'DATA':
                    socket_write($client, "354 End data with <CR><LF>.<CR><LF>\r\n");
                    $data_mode = true;
                    break;
                case 'QUIT':
                    socket_write($client, "221 2.0.0 Bye\r\n");
                    socket_close($client);
                    echo "Client disconnected.\n";
                    goto next_client; // 跳出内层循环，接受下一个客户端
                default:
                    socket_write($client, "502 Command not implemented\r\n");
                    break;
            }
        }
    }
    next_client:
}

socket_close($socket);
?>

package main

import (
    "bufio"
    "fmt"
    "io"
    "net"
    "strings"
    "time"
)

// handleConnection 处理单个客户端连接
func handleConnection(conn net.Conn) {
    defer conn.Close()
    fmt.Printf("Client connected from %s\n", conn.RemoteAddr())

    // 发送欢迎信息
    io.WriteString(conn, "220 example.com ESMTP Service Ready\r\n")

    scanner := bufio.NewScanner(conn)
    mailFrom := ""
    rcptTo := []string{}
    dataMode := false
    mailContent := ""

    for scanner.Scan() {
        line := scanner.Text()
        fmt.Printf("Received from %s: %s\n", conn.RemoteAddr(), line)

        if dataMode {
            // 在DATA模式下，所有行都是邮件内容的一部分，直到遇到单独的'.'
            if line == "." {
                dataMode = false
                fmt.Printf("Received Mail from: %s to: %v\n", mailFrom, rcptTo)
                // 实际应用中，这里会将 mailContent 存储到文件或数据库
                fmt.Printf("Mail Content (truncated):\n%s...\n", mailContent[:min(len(mailContent), 200)])
                io.WriteString(conn, "250 Ok: queued as 12345\r\n")
                mailFrom = ""
                rcptTo = []string{}
                mailContent = ""
            } else {
                mailContent += line + "\r\n"
            }
        } else {
            parts := strings.SplitN(line, " ", 2)
            command := strings.ToUpper(parts[0])
            args := ""
            if len(parts) > 1 {
                args = parts[1]
            }

            switch command {
            case "EHLO", "HELO":
                io.WriteString(conn, "250-example.com\r\n250-PIPELINING\r\n250-SIZE 10240000\r\n250-VRFY\r\n250-AUTH PLAIN LOGIN\r\n250-ENHANCEDSTATUSCODES\r\n250-8BITMIME\r\n250 DSN\r\n")
            case "MAIL":
                if strings.HasPrefix(strings.ToUpper(args), "FROM:<") && strings.HasSuffix(args, ">") {
                    mailFrom = args[6 : len(args)-1]
                    io.WriteString(conn, "250 2.1.0 Ok\r\n")
                } else {
                    io.WriteString(conn, "501 Syntax error in parameters or arguments\r\n")
                }
            case "RCPT":
                if strings.HasPrefix(strings.ToUpper(args), "TO:<") && strings.HasSuffix(args, ">") {
                    rcptTo = append(rcptTo, args[4:len(args)-1])
                    io.WriteString(conn, "250 2.1.5 Ok\r\n")
                } else {
                    io.WriteString(conn, "501 Syntax error in parameters or arguments\r\n")
                }
            case "DATA":
                io.WriteString(conn, "354 End data with <CR><LF>.<CR><LF>\r\n")
                dataMode = true
            case "QUIT":
                io.WriteString(conn, "221 2.0.0 Bye\r\n")
                fmt.Printf("Client disconnected from %s\n", conn.RemoteAddr())
                return // 退出处理函数
            default:
                io.WriteString(conn, "502 Command not implemented\r\n")
            }
        }
    }

    if err := scanner.Err(); err != nil {
        if err != io.EOF { // EOF是正常关闭
            fmt.Printf("Error reading from %s: %v\n", conn.RemoteAddr(), err)
        }
    }
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

func main() {
    port := ":25" // SMTP默认端口
    listener, err := net.Listen("tcp", port)
    if err != nil {
        fmt.Printf("Error listening on %s: %v\n", port, err)
        return
    }
    defer listener.Close()
    fmt.Printf("SMTP Server listening on %s...\n", port)

    for {
        conn, err := listener.Accept()
        if err != nil {
            fmt.Printf("Error accepting connection: %v\n", err)
            time.Sleep(time.Second) // 避免在错误循环中CPU过载
            continue
        }
        go handleConnection(conn) // 为每个新连接启动