#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>

extern int errno ;

int main(argc, argv)
int argc ;
char **argv ;
{
    char c ;
    FILE *fp,*fn ;
    char hostname[64],fromname[64],toname[64],filename[64],sstrs[64],rstrs[2048] ;
    char texts[32000] ;
    char l[64] ;
    register long int i,s ;
    long int m, j, k ;
    struct hostent *hp ;
    struct sockaddr_in sin ;

    /* argc check */
    if ((argc < 4)||(argc > 5)) {
        /* print Usage */
        fprintf(stderr, " Usage: %s smtp_hostname from_name to_name text_filename\n",argv[0]) ;
        exit(-1) ;
    }

    /* set hostname */
    /* gethostname(hostname, sizeof(hostname)) ;*/
    strcpy(hostname,argv[1]) ;
    /* set fromname */
    strcpy(fromname,argv[2]) ;
    /* set toname */
    strcpy(toname,argv[3]) ;
    /* set filename */
    strcpy(filename,argv[4]) ;

    /* open send file */
    if((fn = fopen(filename,"r")) == NULL) {
        perror("cannot open file") ;
        exit(-1) ;
    }

    /* lookup the network address */
    if ((hp = gethostbyname(hostname)) == NULL) {
        fprintf(stderr, "%s: unknown host.\n", hostname) ;
        exit(-1) ;
    }

    /* get a socket to work with. */
    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        fprintf(stderr,"client: socket error.\n") ;
        exit(-1) ;
    }

    /* create the address we will be connecting to. */
    /* use smtp port 25 */
    sin.sin_family = AF_INET ;
    sin.sin_port = htons(25) ;
    bcopy(hp->h_addr, &sin.sin_addr, hp->h_length) ;
    /* try to connect to the address. */
    if (connect(s, &sin, sizeof(sin)) < 0) {
        fprintf(stderr,"client: connect error.\n") ;
        exit(-1) ;
    }

    /* open reading the socket. */
    fp = fdopen(s, "r") ;

    /* first read some strings. */
    fgets(rstrs, 2048, fp) ;

    /* send fromname */
    strcpy(sstrs,"HELO ") ;
    strcat(sstrs,hostname) ;
    strcat(sstrs,"\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;
    printf("Connect to:%s\n",hostname) ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    /* send fromname */
    strcpy(sstrs,"MAIL FROM:<") ;
    strcat(sstrs,fromname) ;
    strcat(sstrs,">\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ; 
    printf("Mail from:%s\n",fromname) ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    /* send toname */
    strcpy(sstrs,"RCPT TO:<") ;
    strcat(sstrs,toname) ;
    strcat(sstrs,">\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;
    printf("Mail to:%s\n",toname) ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    /* put message ready */
    strcpy(sstrs,"DATA\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    /* put message */
    strcpy(sstrs,"From:") ;
    strcat(sstrs,fromname) ;
    strcat(sstrs,"\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;
    strcpy(sstrs,"To:") ;
    strcat(sstrs,toname) ;
    strcat(sstrs,"\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;
    strcpy(sstrs,"X-Mailer:smtpsend ") ;
    strcat(sstrs,"\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;

    printf("---DATA Start---\n") ;
    printf("%s\n------------------------\n",sstrs) ;
  
    i = 0 ;
    while ((m = fgetc(fn)) != EOF)
        texts[i++] = m ;

    j = 0 ; k = 0 ; sstrs[0] = 0 ;
    while (texts[j] != 0) {
        if((texts[j] == 0x0A)||(texts[j] == 0x0D)){
            sstrs[k++] = 0 ;
            j++ ;
            if((texts[j] == 0x0A)||(texts[j] == 0x0D)){
                sstrs[k] = 0 ;
                j++ ;
            }
            printf("%s\n",sstrs) ;
            strcat(sstrs,"\r\n") ;
            send(s,sstrs,strlen(sstrs),0) ;
            k = 0 ; sstrs[0] = 0 ;
         } else {
             sstrs[k++] = texts[j++] ;
         }
    }
    if( k > 0){
        sstrs[k] = 0 ;
        printf("%s\n",sstrs) ;
        strcat(sstrs,"\r\n") ;
        send(s,sstrs,strlen(sstrs),0) ;
    }

    /* put message end */
    strcpy(sstrs,"\r\n.\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;
    printf("------------------------\n") ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    /* quit smtp server*/
    strcpy(sstrs,"QUIT\r\n") ;
    send(s,sstrs,strlen(sstrs), 0) ;

    /* read response */
    fgets(rstrs, 2048, fp) ;

    close(s) ;
    fclose(fp) ;
    fclose(fn) ;

    printf("---Message Send---\n") ;

    exit(0) ;
}

   The second step in the procedure is the RCPT command.

      RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>

   The first or only argument to this command includes a forward-path
   (normally a mailbox and domain, always surrounded by "<" and ">"
   brackets) identifying one recipient.