The following is the set of commands from the SMTP RFC to be sent to the SMTP server to make it listen and accept our e-mails to be sent to the recipients.

Connect to SMTP server using port 25
                Expect 220
                Send HELO myFQDN
                Expect 250
                Send MAIL FROM:<From-mail@myFQDN.com>
                Expect 250
                Send RCPT TO:<mail_address>
                Expect 250
                Send DATA
                Expect 354
                Send body of message
                Send <CRLF>.<CRLF>
                Expect 250
                Send QUIT
                Expect 221
                Close connection

   The above is the way an SMTP client and a SMTP server interact between them. All the above strings sent to the Command Line SMTP program, including the e-mail addresses should be appended with \r\n. Otherwise it will not be considered as the end of the line.

   Also the received data will contain numbers like 250, 354 etc., in the front. These numbers mean either the messages is accepted or rejected or unrecognized. 500 is the error returned if the command is unrecognized.

Command Line SMTP sample using Win32:

   The following sample program can be used to send an e-mail to the SMTP server.

      #include <winsock2.h>
      #include <windows.h>
      #include <iostream.h>

      #define MAX_LENGTH 1024

      void main()
      {
          int s_len, r_len;
          int skt_Smtp;
          int success;
          struct sockaddr_in st_Sockaddr;
          char recv_Buf[MAX_LENGTH];
          char send_Buf[MAX_LENGTH];

          //Initialize Sockets
          WSADATA wsa;
          WSAStartup(MAKEWORD(2, 0), &wsa);


          skt_Smtp = socket(AF_INET,SOCK_STREAM,0);
          if (skt_Smtp < 0)
          {
              cout<< "Error Creating Socket"<<endl;
              return;
          }
         else
         {
              st_Sockaddr.sin_family = AF_INET;
              st_Sockaddr.sin_port = htons(25);

             //Get the IP address and initialize the structure
             st_Sockaddr.sin_addr.s_addr = inet_addr("IPAddress");

             success = connect(skt_Smtp,(struct sockaddr *) &st_Sockaddr,sizeof(st_Sockaddr));

             r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
             recv_Buf[r_len] = '\0';

             cout<< recv_Buf<<endl;

             //Say Hello to the domain
             strcpy(send_Buf,"HELO sampledomain.com\r\n");

             s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

             r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
             recv_Buf[r_len] = '\0';

             cout<< recv_Buf<<endl;

             //Send from address
             strcpy(send_Buf ,  "MAIL FROM: frome-mailid@sampledomain.com\r\n");

             s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

             r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
             recv_Buf[r_len] = '\0';

             cout<< recv_Buf<<endl;
 
             //Send RCPT address
             strcpy(send_Buf,"RCPT TO:<toemail-id@sampledomain.com>\r\n");

             s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

             r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
             recv_Buf[r_len] = '\0';

             cout<< recv_Buf<<endl;

             // Send DATA
             strcpy(send_Buf,"DATA\r\n");

             s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

             r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
             recv_Buf[r_len] = '\0';

             cout<< recv_Buf<<endl;


             // Send DATA
            strcpy(send_Buf,"Subject:Test Win32 command line SMTP Subject\r\n\r\n");

            s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);


            //Send body
            strcpy(send_Buf,"Test e-mail for Win32 command line SMTP\r\n.\r\n");

            s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

            r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
            recv_Buf[r_len] = '\0';

            cout<< recv_Buf<<endl;

            //Send QUIT
            strcpy(send_Buf,"QUIT\n");

            s_len = send(skt_Smtp,send_Buf,strlen(send_Buf),0);

            r_len = recv(skt_Smtp,recv_Buf,MAX_LENGTH,0);
            recv_Buf[r_len] = '\0';
 
            cout<< recv_Buf<<endl;

            closesocket(skt_Smtp);
         }

         WSACleanup();
     }

   This program can be compiled as a console application. The items highlighted in BOLD and BLUE colors should be replaced with the right names.

Note: As this program uses sockets, this needs to be linked with ws2_32.lib library. Refer for more information on socket client.