출처 : http://www.cplusplus.com/forum/windows/35333/

#include "io.h"
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>

#include "winsock2.h"      /* WSAGetLastError, WSAStartUp  */
#define snprintf _snprintf


static void sendmail_write(
                const int  sock,
                const char *str,
                const char *arg
            ) {
    char buf[4096];

    if (arg != NULL)
        snprintf(buf, sizeof(buf), str, arg);
    else
        snprintf(buf, sizeof(buf), str);

    send(sock, buf, strlen(buf), 0);
}

static int sendmail(
                const char *from,
                const char *to,
                const char *subject,
                const char *body,
                const char *hostname,
                const int   port
            ) {

    struct hostent *host;
    struct sockaddr_in saddr_in;
    int sock = 0;


    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        return -1;
    }


    sock = socket(AF_INET, SOCK_STREAM, 0);
    host = gethostbyname(hostname);

    saddr_in.sin_family      = AF_INET;
    saddr_in.sin_port        = htons((u_short)port);
    saddr_in.sin_addr.s_addr = 0;

    memcpy((char*)&(saddr_in.sin_addr), host->h_addr, host->h_length);

    if (connect(sock, (struct sockaddr*)&saddr_in, sizeof(saddr_in)) == -1) {
        return -2;
    }

    sendmail_write(sock, "HELO %s\n",       from);    // greeting
    sendmail_write(sock, "MAIL FROM: %s\n", from);    // from
    sendmail_write(sock, "RCPT TO: %s\n",   to);      // to
    sendmail_write(sock, "DATA\n",          NULL);    // begin data

    // next comes mail headers
    sendmail_write(sock, "From: %s\n",      from);
    sendmail_write(sock, "To: %s\n",        to);
    sendmail_write(sock, "Subject: %s\n",   subject);

    sendmail_write(sock, "\n",              NULL);

    sendmail_write(sock, "%s\n",            body);    // data

    sendmail_write(sock, ".\n",             NULL);    // end data
    sendmail_write(sock, "QUIT\n",          NULL);    // terminate

    close(sock);

    return 0;
}


int main(int argc, char *argv[]) {

    int ret = sendmail(
        "testas@domainname.com",  // from
        "Null665@gmail.com", // to
        "Subject",
        "body",
        "smtp.google.com",
        25
    );

    if (ret != 0)
        fprintf(stderr, "Failed to send mail (code: %i).\n", ret);
    else
        fprintf(stdout, "Mail successfully sent.\n");

    return ret;
}

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