C语言利用SMTP协议发送邮件

 #ifdef WIN32
 #include <windows.h>
 #include <stdio.h>
 #else

 #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>
 #endif
 struct data6
 {
     unsigned int d4:;
     unsigned int d3:;
     unsigned int d2:;
     unsigned int d1:;
 };
 // 协议中加密部分使用的是base64方法
 char con628(char c6);
 void base64(char *dbuf,char *buf128,int len);
 void sendemail(char *email,char *body);
 int open_socket(struct sockaddr *addr);

 int main()
 {
     char email[] = "987654321@qq.com";
     char body[] = "From: \"lucy\"<123456789@qq.com>\r\n"
     "To: \"dasiy\"<987654321@qq.com>\r\n"
     "Subject: Hello\r\n\r\n"
     "Hello World, Hello Email!";
     sendemail(email, body);
     return ;
 }

 char con628(char c6)
 {
     char rtn = '\0';
     if (c6 < ) rtn = c6 + ;
     else if (c6 < ) rtn = c6 + ;
     else if (c6 < ) rtn = c6 - ;
     else if (c6 == ) rtn = ;
     else rtn = ;
     return rtn;
 }

 // base64的实现
 void base64(char *dbuf, char *buf128, int len)
 {
     struct data6 *ddd = NULL;
     int i = ;
     char buf[] = {};
     char *tmp = NULL;
     char cc = '\0';
     memset(buf, , );
     strcpy(buf, buf128);
     for(i = ; i <= len/; i++)
     {
     tmp = buf+(i-)*;
     cc = tmp[];
     tmp[] = tmp[];
     tmp[] = cc;
     ddd = (struct data6 *)tmp;
     dbuf[(i-)*+] = con628((unsigned int)ddd->d1);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d2);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d3);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d4);
     }
     if(len% == )
     {
     tmp = buf+(i-)*;
     cc = tmp[];
     tmp[] = tmp[];
     tmp[] = cc;
     ddd = (struct data6 *)tmp;
     dbuf[(i-)*+] = con628((unsigned int)ddd->d1);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d2);
     dbuf[(i-)*+] = '=';
     dbuf[(i-)*+] = '=';
     }
     if(len% == )
     {
     tmp = buf+(i-)*;
     cc = tmp[];
     tmp[] = tmp[];
     tmp[] = cc;
     ddd = (struct data6 *)tmp;
     dbuf[(i-)*+] = con628((unsigned int)ddd->d1);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d2);
     dbuf[(i-)*+] = con628((unsigned int)ddd->d3);
     dbuf[(i-)*+] = '=';
     }
     return;
 }
 // 发送邮件
 void sendemail(char *email, char *body)
 {
     int sockfd = ;
     struct sockaddr_in their_addr = {};
     char buf[] = {};
     char rbuf[] = {};
     char login[] = {};
     char pass[] = {};
     #ifdef WIN32
     WSADATA WSAData;
     WSAStartup(MAKEWORD(, ), &WSAData);
     #endif
     memset(&their_addr, , sizeof(their_addr));
     their_addr.sin_family = AF_INET;
     their_addr.sin_port = htons();
     their_addr.sin_addr.s_addr = inet_addr("112.90.141.71");//qq smtp 服务器
     // 连接邮件服务器，如果连接后没有响应，则2 秒后重新连接
     sockfd = open_socket((struct sockaddr *)&their_addr);
     memset(rbuf,,);
     while(recv(sockfd, rbuf, , ) == )
     {
         printf("reconnect...\n");
         Sleep();
         //close(sockfd);
         sockfd = open_socket((struct sockaddr *)&their_addr);

         memset(rbuf,,);
     }

     printf("%s\n", rbuf);

     // EHLO
     memset(buf, , );
     sprintf(buf, "EHLO abcdefg-PC\r\n");
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // AUTH LOGIN
     memset(buf, , );
     sprintf(buf, "AUTH LOGIN\r\n");
     send(sockfd, buf, strlen(buf), );
     printf("%s\n", buf);
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // USER
     memset(buf, , );
     sprintf(buf,"");//你的qq号
     memset(login, , );
     base64(login, buf, strlen(buf));
     sprintf(buf, "%s\r\n", login);
     send(sockfd, buf, strlen(buf), );
     printf("%s\n", buf);
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // PASSWORD
     sprintf(buf, "qqpassword");//你的qq密码
     memset(pass, , );
     base64(pass, buf, strlen(buf));
     sprintf(buf, "%s\r\n", pass);
     send(sockfd, buf, strlen(buf), );
     printf("%s\n", buf);

     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // MAIL FROM
     memset(buf, , );
     sprintf(buf, "MAIL FROM: <123456789@qq.com>\r\n");
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // RCPT TO 第一个收件人
     sprintf(buf, "RCPT TO:<%s>\r\n", email);
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // DATA 准备开始发送邮件内容
     sprintf(buf, "DATA\r\n");
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // 发送邮件内容，\r\n.\r\n内容结束标记
     sprintf(buf, "%s\r\n.\r\n", body);
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

     // QUIT
     sprintf(buf, "QUIT\r\n");
     send(sockfd, buf, strlen(buf), );
     memset(rbuf, , );
     recv(sockfd, rbuf, , );
     printf("%s\n", rbuf);

 // VC2005 需要使用
     closesocket(sockfd);
 //    close(sockfd);
     #ifdef WIN32
     WSACleanup();
     #endif
     return;

 }
     // 打开TCP Socket连接
 int open_socket(struct sockaddr *addr)
 {
         int sockfd = ;
         sockfd=socket(PF_INET, SOCK_STREAM, );
         if(sockfd < )
         {
             fprintf(stderr, "Open sockfd(TCP) error!\n");
             exit(-);
         }
         if(connect(sockfd, addr, sizeof(struct sockaddr)) < )
         {
             fprintf(stderr, "Connect sockfd(TCP) error!\n");
             exit(-);
         }
         return sockfd;
 } 

出处：http://blog.csdn.net/zx824/article/details/7387418