How to send email in C?

You can use the cURL library if you want to send mail via SMTP.

Here is an example taken from their site:

  #include <stdio.h>
  #include <string.h>
  #include <curl/curl.h>

  int main(void)
  {
    CURL *curl;
    CURLcode res;
    struct curl_slist *recipients = NULL;

    /* value for envelope reverse-path */ 
    static const char *from = "<[email protected]>";

    /* this becomes the envelope forward-path */ 
    static const char *to = "<[email protected]>";

    curl = curl_easy_init();
    if(curl) {
      /* this is the URL for your mailserver - you can also use an smtps:// URL
       * here */ 
      curl_easy_setopt(curl, CURLOPT_URL, "smtp://mail.example.net.");

      /* Note that this option isn't strictly required, omitting it will result in
       * libcurl will sent the MAIL FROM command with no 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);

      /* Note that the CURLOPT_MAIL_RCPT takes a list, not a char array.  */ 
      recipients = curl_slist_append(recipients, to);
      curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);

      /* You provide the payload (headers and the body of the message) as the
       * "data" element. There are two choices, either:
       * - provide a callback function and specify the function name using the
       * CURLOPT_READFUNCTION option; or
       * - just provide a FILE pointer that can be used to read the data from.
       * The easiest case is just to read from standard input, (which is available
       * as a FILE pointer) as shown here.
       */ 
      curl_easy_setopt(curl, CURLOPT_READDATA, stdin);
      curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

      /* send the message (including headers) */ 
      res = curl_easy_perform(curl);
      /* Check for errors */ 
      if(res != CURLE_OK)
        fprintf(stderr, "curl_easy_perform() failed: %s\n",
                curl_easy_strerror(res));

      /* free the list of recipients */ 
      curl_slist_free_all(recipients);

      /* curl won't send the QUIT command until you call cleanup, so you should be
       * able to re-use this connection for additional messages (setting
       * CURLOPT_MAIL_FROM and CURLOPT_MAIL_RCPT as required, and calling
       * curl_easy_perform() again. It may not be a good idea to keep the
       * connection open for a very long time though (more than a few minutes may
       * result in the server timing out the connection), and you do want to clean
       * up in the end.
       */ 
      curl_easy_cleanup(curl);
          }
        return 0;
    }

Example link: link