前言

C语言实现SMTP发邮件（SSL协议）
这里作者采用OpenSSL来发送和解析SSL协议，并实验SMTP发邮件
以网易云smtp.163.com为例

引用库

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#include <unistd.h>
#include <sys/time.h>
#include <openssl/ssl.h>
#include <openssl/err.h>
#include <assert.h>
#include <netdb.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>

示例代码

#define SMTP_PORT 465
#define SMTP_HOST ("smtp.163.com")
#define SMTP_USER ("XXXXX") // base64编码
#define SMTP_PAWD ("XXXXX") // base64编码

int sendMessage(char *message, char *form, char *to)
{
    struct hostent *host = NULL;
    struct sockaddr_in serverAddr;
    SSL_CTX *ctx = NULL;
    SSL *ssl = NULL;
    char buf[320] = {0};
    char *messagebuf = NULL;
    char ip[32] = {0};
    int sockfd = 0;

    host = gethostbyname(SMTP_HOST);
    if (host == NULL)
    {
        return -1;
    }
    ctx = SSL_CTX_new(SSLv23_client_method());
    if (ctx == NULL)
    {
        return -1;
    }

    inet_ntop(host->h_addrtype, host->h_addr_list[0], ip, sizeof(ip));

    memset(&serverAddr, 0, sizeof(struct sockaddr_in));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(SMTP_PORT);
    serverAddr.sin_addr.s_addr = inet_addr(ip);
    sockfd = socket(PF_INET, SOCK_STREAM, 0);

    if (sockfd < 0)
    {
        return -1;
    }

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

    ssl = SSL_new(ctx);
    SSL_set_fd(ssl, sockfd);

    if (SSL_connect(ssl) != 1)
    {
        return -1;
    }

    // HELO
    sprintf(buf, "%s", "EHLO smtp.163.com \r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // AUTH LOGIN 身份认证
    sprintf(buf, "%s", "AUTH LOGIN\r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送账号
    sprintf(buf, "%s\r\n", SMTP_USER);
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送密码
    sprintf(buf, "%s\r\n", SMTP_PAWD);
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送发件人
    sprintf(buf, "%s", "MAIL FROM: <");
    strcat(buf, form);
    strcat(buf, ">\r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送收件人
    sprintf(buf, "%s", "RCPT TO:<");
    strcat(buf, to);
    strcat(buf, ">\r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送邮件内容标识
    sprintf(buf, "%s", "DATA\r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    // 发送邮件
    while (1)
    {
        tmp = (char *)calloc(sizeof(char), 13 + strlen(box));
        if(tmp){
            break;
        }
        sleep(1);
    }
    sprintf(messagebuf, "%s\r\n.\r\n", message);
    SSL_write(ssl, messagebuf, strlen(messagebuf));
    if (SSL_read(ssl, messagebuf, strlen(messagebuf)) <= 0)
    {
        return -1;
    }
    free(messagebuf);

    // 断开连接
    sprintf(buf, "%s", "QUIT\r\n");
    SSL_write(ssl, buf, strlen(buf));
    if (SSL_read(ssl, buf, 320) <= 0)
    {
        return -1;
    }

    close(sockfd);
    SSL_shutdown(ssl);
    SSL_free(ssl);
    SSL_CTX_free(ctx);
    return 0;
}

int main()
{
    char message[] = "From: test<XXX@163.com>\r\n"
                     "TO: test<XXX@qq.com>\r\n"
                     "Subject: test\r\n\r\n"
                     "hello world\r\n";
    sendMessage(message, "XXX@163.com", "XXX@qq.com");
}

编译命令

GCC：

gcc ./sendMessage.c -o sendMessage.out -lssl