用c++封装smtp协议

以下是用C语言封装SMTP协议的示例代码（仅供参考）：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>

#define BUFFER_SIZE 1024

void send_command(int sockfd, const char* command, const char* args) {
    char buffer[BUFFER_SIZE];
    snprintf(buffer, BUFFER_SIZE, "%s %s\r\n", command, args);
    send(sockfd, buffer, strlen(buffer), 0);
}

int recv_response(int sockfd) {
    char buffer[BUFFER_SIZE];
    int n = recv(sockfd, buffer, BUFFER_SIZE - 1, 0);
    if (n < 0) {
        perror("recv");
        return -1;
    }
    buffer[n] = '\0';
    printf("%s", buffer);
    return atoi(buffer);
}

int send_mail(const char* server, const char* from, const char* to, const char* subject, const char* body) {
    struct hostent* host = gethostbyname(server);
    if (host == NULL) {
        printf("Unknown host: %s\n", server);
        return -1;
    }

    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("socket");
        return -1;
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(25);
    server_addr.sin_addr = *((struct in_addr*) host->h_addr);

    if (connect(sockfd, (struct sockaddr*) &server_addr, sizeof(server_addr)) < 0) {
        perror("connect");
        return -1;
    }

    recv_response(sockfd); // 220

    send_command(sockfd, "HELO", server);
    if (recv_response(sockfd) != 250) {
        printf("Failed to send HELO\n");
        return -1;
    }

    send_command(sockfd, "MAIL FROM:", from);
    if (recv_response(sockfd) != 250) {
        printf("Failed to send MAIL FROM\n");
        return -1;
    }

    send_command(sockfd, "RCPT TO:", to);
    if (recv_response(sockfd) != 250) {
        printf("Failed to send RCPT TO\n");
        return -1;
    }

    send_command(sockfd, "DATA", "");
    if (recv_response(sockfd) != 354) {
        printf("Failed to send DATA\n");
        return -1;
    }

    send_command(sockfd, "Subject:", subject);
    send_command(sockfd, "From:", from);
    send_command(sockfd, "To:", to);
    send_command(sockfd, "", "");
    send_command(sockfd, body, "");

    send_command(sockfd, ".", "");
    if (recv_response(sockfd) != 250) {
        printf("Failed to send .\n");
        return -1;
    }

    send_command(sockfd, "QUIT", "");
    if (recv_response(sockfd) != 221) {
        printf("Failed to send QUIT\n");
        return -1;
    }

    close(sockfd);
    return 0;
}

int main(int argc, char* argv[]) {
    if (argc < 6) {
        printf("Usage: %s server from to subject body\n", argv[0]);
        return -1;
    }
    const char* server = argv[1];
    const char* from = argv[2];
    const char* to = argv[3];
    const char* subject = argv[4];
    const char* body = argv[5];
    return send_mail(server, from, to, subject, body);
}

这个示例代码实现了一个简单的SMTP客户端，可以向指定的服务器发送邮件。具体步骤如下：

1. 解析服务器的IP地址
2. 创建TCP连接并发送HELO命令
3. 发送MAIL FROM命令
4. 发送RCPT TO命令
5. 发送DATA命令
6. 发送邮件头和正文
7. 发送.命令
8. 发送QUIT命令

在实际使用中，需要根据具体的SMTP服务器要求修改代码。例如，有些服务器需要验证用户名和密码，需要在发送MAIL FROM命令之前发送AUTH LOGIN命令。另外，需要注意邮件正文中的换行符应该使用\r\n，而不是\n。