用C语言，使用winscok库，编写一个通过SMTP协议的发送邮件的文件

好的，以下是一个使用C语言和Winsock库发送邮件的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h>

#pragma comment(lib, "ws2_32.lib")  // 链接Winsock库

#define SERVER_ADDR "smtp.example.com"  // SMTP服务器地址
#define SERVER_PORT 25                  // SMTP服务器端口号
#define FROM_ADDR   "sender@example.com"  // 发件人邮箱地址
#define TO_ADDR     "recipient@example.com"  // 收件人邮箱地址
#define USERNAME    "sender@example.com"  // 发件人用户名
#define PASSWORD    "password"             // 发件人密码

int main()
{
    WSADATA wsaData;
    SOCKET sock;
    struct sockaddr_in serverAddr;
    char buf[1024];
    int ret;

    // 初始化Winsock库
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        printf("WSAStartup failed!\n");
        return 1;
    }

    // 创建套接字
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock == INVALID_SOCKET) {
        printf("socket failed: %d\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    // 设置服务器地址和端口号
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr(SERVER_ADDR);
    serverAddr.sin_port = htons(SERVER_PORT);

    // 连接服务器
    if (connect(sock, (struct sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
        printf("connect failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的欢迎信息
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送HELO命令
    sprintf(buf, "HELO %s\r\n", SERVER_ADDR);
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送AUTH LOGIN命令
    sprintf(buf, "AUTH LOGIN\r\n");
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送用户名
    sprintf(buf, "%s\r\n", USERNAME);
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送密码
    sprintf(buf, "%s\r\n", PASSWORD);
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送MAIL FROM命令
    sprintf(buf, "MAIL FROM:<%s>\r\n", FROM_ADDR);
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送RCPT TO命令
    sprintf(buf, "RCPT TO:<%s>\r\n", TO_ADDR);
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送DATA命令
    sprintf(buf, "DATA\r\n");
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送邮件内容
    sprintf(buf, "From: <%s>\r\n", FROM_ADDR);
    send(sock, buf, strlen(buf), 0);
    sprintf(buf, "To: <%s>\r\n", TO_ADDR);
    send(sock, buf, strlen(buf), 0);
    sprintf(buf, "Subject: Test\r\n");
    send(sock, buf, strlen(buf), 0);
    sprintf(buf, "\r\n");
    send(sock, buf, strlen(buf), 0);
    sprintf(buf, "This is a test email.\r\n");
    send(sock, buf, strlen(buf), 0);
    sprintf(buf, ".\r\n");
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 发送QUIT命令
    sprintf(buf, "QUIT\r\n");
    ret = send(sock, buf, strlen(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("send failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }

    // 接收服务器的响应
    ret = recv(sock, buf, sizeof(buf), 0);
    if (ret == SOCKET_ERROR) {
        printf("recv failed: %d\n", WSAGetLastError());
        closesocket(sock);
        WSACleanup();
        return 1;
    }
    buf[ret] = '\0';
    printf("Server: %s\n", buf);

    // 关闭套接字和Winsock库
    closesocket(sock);
    WSACleanup();

    return 0;
}

请将上述代码中的`SERVER_ADDR`、`SERVER_PORT`、`FROM_ADDR`、`TO_ADDR`、`USERNAME`和`PASSWORD`分别替换为你的SMTP服务器地址、SMTP服务器端口号、发件人邮箱地址、收件人邮箱地址、发件人用户名和密码。