curl / libcurl / API / Examples / smtp-ssl.c


 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 * Copyright (C) Daniel Stenberg, <>, et al.
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 * SPDX-License-Identifier: curl
/* <DESC>
 * Send SMTP email using implicit SSL
 * </DESC>
#include <stdio.h>
#include <string.h>
#include <curl/curl.h>
/* This is a simple example showing how to send mail using libcurl's SMTP
 * capabilities. It builds on the smtp-mail.c example to add authentication
 * and, more importantly, transport security to protect the authentication
 * details from being snooped.
 * Note that this example requires libcurl 7.20.0 or above.
#define FROM_MAIL     "<>"
#define TO_MAIL       "<>"
#define CC_MAIL       "<>"
static const char *payload_text =
  "Date: Mon, 29 Nov 2010 21:54:29 +1100\r\n"
  "To: " TO_MAIL "\r\n"
  "From: " FROM_MAIL "\r\n"
  "Cc: " CC_MAIL "\r\n"
  "Message-ID: <dcd7cb36-11db-487a-9f3a-e652a9458efd@"
  "Subject: SMTP example message\r\n"
  "\r\n" /* empty line to divide headers from body, see RFC5322 */
  "The body of the message starts here.\r\n"
  "It could be a lot of lines, could be MIME encoded, whatever.\r\n"
  "Check RFC5322.\r\n";
struct upload_status {
  size_t bytes_read;
static size_t payload_source(char *ptr, size_t size, size_t nmemb, void *userp)
  struct upload_status *upload_ctx = (struct upload_status *)userp;
  const char *data;
  size_t room = size * nmemb;
  if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
    return 0;
  data = &payload_text[upload_ctx->bytes_read];
  if(data) {
    size_t len = strlen(data);
    if(room < len)
      len = room;
    memcpy(ptr, data, len);
    upload_ctx->bytes_read += len;
    return len;
  return 0;
int main(void)
  CURL *curl;
  CURLcode res = CURLE_OK;
  struct curl_slist *recipients = NULL;
  struct upload_status upload_ctx = { 0 };
  curl = curl_easy_init();
  if(curl) {
    /* Set username and password */
    curl_easy_setopt(curl, CURLOPT_USERNAME, "user");
    curl_easy_setopt(curl, CURLOPT_PASSWORD, "secret");
    /* This is the URL for your mailserver. Note the use of smtps:// rather
     * than smtp:// to request a SSL based connection. */
    curl_easy_setopt(curl, CURLOPT_URL, "smtps://");
    /* If you want to connect to a site who is not using a certificate that is
     * signed by one of the certs in the CA bundle you have, you can skip the
     * verification of the server's certificate. This makes the connection
     * If you have a CA cert for the server stored someplace else than in the
     * default bundle, then the CURLOPT_CAPATH option might come handy for
     * you. */
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
    /* If the site you are connecting to uses a different host name that what
     * they have mentioned in their server certificate's commonName (or
     * subjectAltName) fields, libcurl will refuse to connect. You can skip
     * this check, but this will make the connection less secure. */
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
    /* Note that this option is not strictly required, omitting it will result
     * in libcurl sending the MAIL FROM command with empty sender data. All
     * autoresponses should have an empty reverse-path, and should be directed
     * to the address in the reverse-path which triggered them. Otherwise,
     * they could cause an endless loop. See RFC 5321 Section 4.5.5 for more
     * details.
    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM_MAIL);
    /* Add two recipients, in this particular case they correspond to the
     * To: and Cc: addressees in the header, but they could be any kind of
     * recipient. */
    recipients = curl_slist_append(recipients, TO_MAIL);
    recipients = curl_slist_append(recipients, CC_MAIL);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
    /* We are using a callback function to specify the payload (the headers and
     * body of the message). You could just use the CURLOPT_READDATA option to
     * specify a FILE pointer to read from. */
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
    curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
    /* Since the traffic will be encrypted, it is very useful to turn on debug
     * information within libcurl to see what is happening during the
     * transfer */
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
    /* Send the message */
    res = curl_easy_perform(curl);
    /* Check for errors */
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
    /* Free the list of recipients */
    /* Always cleanup */
  return (int)res;


This source code example is simplified and ignores return codes and error checks to a large extent. We do this to highlight the libcurl function calls and related options and reduce unrelated code.

A real-world application will of course properly check every return value and exit correctly at the first serious error.