#define MAX_CH 255
#define POP3_PORT 110
 
int sockfd;
struct sockaddr_in servaddr;
struct hostent* h;
char domain[MAX_CH];
/*buffers for sending/receiving messages*/
char sendbuf[MAXLEN]; 
char recvbuf[MAXLEN];
 
/* user input's the domain name*/
printf("POP3 server: ");
scanf("%s", domain);
 
/*create socket */
if ( (sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) 
{
	printf("Error creating socket.\n");
	exit(EXIT_FAILURE);
}  
 
/* for the socket to "understand" the domain name
    it must be converted using the gethostbyname() 
    function */
h = gethostbyname(domain);
 
/* fill servaddr with zeroes, and then initialize it */
memset((char *) &servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
memcpy((char *)&servaddr.sin_addr.s_addr,
              (char *)h->h_addr, 
               h->h_length);
servaddr.sin_port = htons(POP3_PORT);
 
/* connect to the server  */
if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0 ) 
{
    printf("Server could not be reached.\n");
    exit(EXIT_FAILURE);
}
 
/* read the response from the server
    and verify it's ok */	
read(sockfd,recvbuf,sizeof(recvbuf));
if(!strstr(recvbuf,"+OK"))
	exit(EXIT_FAILURE);

/* read the username from the user
    and send the command to the server */
scanf("%s", username);
sprintf(sendbuf,"USER %s\n", username);
write(sockfd,sendbuf,strlen(sendbuf));
 
/*verify that the server responded OK*/
memset(recvbuf,0,sizeof(recvbuf));
read(sockfd,recvbuf,sizeof(recvbuf));
if(!strstr(recvbuf,"+OK")) 
{
	printf("Incorrect user name.\n");
	exit(EXIT_FAILURE);
}

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

memset(sendbuf,0,sizeof(sendbuf));
sprintf(sendbuf,"QUIT\n");
write(sockfd,sendbuf,strlen(sendbuf));
close(sockfd);