Docs: add simple SMTP example

Add a simple SMTP example program, patterned after some of the existing
examples, and the curl application.

This version addresses issues raised by David Woodhouse on comments in
the simplesmtp.c example.

docs/examples/Makefile.inc

@@ -4,7 +4,7 @@ check_PROGRAMS = 10-at-a-time anyauthput cookie_interface debug fileupload \
   https multi-app multi-debugcallback multi-double multi-post multi-single \
   persistant post-callback postit2 sepheaders simple simplepost simplessl  \
   sendrecv httpcustomheader certinfo chkspeed ftpgetinfo ftp-wildcard \
-  smtp-multi
+  smtp-multi simplesmtp
 
 # These examples require external dependencies that may not be commonly
 # available on POSIX systems, so don't bother attempting to compile them here.

docs/examples/simplesmtp.c

+/*****************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ */
+
+#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 = "bradh@example.com";
+
+  /* this becomes the envelope forward-path */
+  static const char *to = "bradh@example.net";
+
+  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);
+
+    /* send the message (including headers) */
+    res = curl_easy_perform(curl);
+
+    /* 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;
+}