SMTP Client in C

What is SMTP?

SMTP Session Flow

Implementing It in C

Helper Functions

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

#define BUF_SIZE 4096

// send a command and print the server's response
void smtp_command(int sock, const char *cmd) {
    char buf[BUF_SIZE];
    if (cmd != NULL) {
        send(sock, cmd, strlen(cmd), 0);
        printf("C: %s", cmd);
    }
    int n = recv(sock, buf, BUF_SIZE - 1, 0);
    if (n > 0) {
        buf[n] = '\0';
        printf("S: %s", buf);
    }
}

// connect to an SMTP server
int connect_smtp(const char *host, int port) {
    struct hostent *h = gethostbyname(host);
    if (!h) { fprintf(stderr, "Host lookup failed\n"); return -1; }

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) { perror("socket"); return -1; }

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    memcpy(&addr.sin_addr.s_addr, h->h_addr, h->h_length);

    if (connect(sock, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        perror("connect"); close(sock); return -1;
    }
    return sock;
}

Main Logic

int main(void) {
    const char *smtp_host = "mail.example.com";
    const char *from = "alice@example.com";
    const char *to   = "bob@example.com";

    // 1. connect
    int sock = connect_smtp(smtp_host, 25);
    if (sock < 0) return 1;

    // 2. read greeting
    smtp_command(sock, NULL);

    // 3. SMTP session
    smtp_command(sock, "HELO mycomputer\r\n");

    char cmd[BUF_SIZE];

    snprintf(cmd, BUF_SIZE, "MAIL FROM:<%s>\r\n", from);
    smtp_command(sock, cmd);

    snprintf(cmd, BUF_SIZE, "RCPT TO:<%s>\r\n", to);
    smtp_command(sock, cmd);

    smtp_command(sock, "DATA\r\n");

    // 4. headers and body
    snprintf(cmd, BUF_SIZE,
        "Subject: Test from C program\r\n"
        "From: %s\r\n"
        "To: %s\r\n"
        "\r\n"                            // blank line ends headers
        "Hello!\r\n"
        "This mail was sent from a C program.\r\n"
        ".\r\n",                          // line with only a period ends the body
        from, to);
    smtp_command(sock, cmd);

    // 5. end session
    smtp_command(sock, "QUIT\r\n");

    close(sock);
    printf("\nMail sent.\n");
    return 0;
}

Feature Extensions

Extension 1: Read the body from the keyboard

// read body from the keyboard
printf("Enter mail body (blank line to finish):\n");
char body[BUF_SIZE] = "";
char line[256];
while (1) {
    fgets(line, 256, stdin);
    if (line[0] == '\n') break;  // blank line to finish
    strcat(body, line);
}

Extension 2: Address book

// address book
const char *addressbook[] = {
    "user1@example.com",
    "user2@example.com",
    "user3@example.com"
};
int num_addr = 3;

printf("Select recipient:\n");
for (int i = 0; i < num_addr; i++)
    printf("  %d: %s\n", i + 1, addressbook[i]);

int choice;
scanf("%d", &choice);
const char *to = addressbook[choice - 1];

Extension 3: Load body from a file

// read body from a text file
FILE *fp = fopen("message.txt", "r");
if (fp == NULL) { perror("fopen"); return 1; }

char body[BUF_SIZE] = "";
char line[256];
while (fgets(line, 256, fp) != NULL)
    strcat(body, line);
fclose(fp);

Challenges