custom MailSystemInterface implementation I need to create a custom implementation of MailSystemInterface.

I created a file called my_module.mail.inc but I don't know how to load it and how to tell Drupal to use it by default.

I installed the mailsystem and autoload modules but don't know what to do next.

Any help would be greatly appreciated. Thanks
for mailsystem to work, you need to implement MailSystemInterface class and enable your new mailsystem class as default, for example in module hook_install():

    variable_set('mail_system', array_merge(
       variable_get('mail_system', array('default-system' =&gt; 'DefaultMailSystem')), // Previously set mail_system variable
       array('login' =&gt; 'MyCustomMailClass')   // My new key(s) which ADD to the previous keys
    );


Note: array('login' =&gt; 'MyCustomMailClass') "login" is a module name, which will be used to identify your new mailinterface.

Next you should call your new mailsystem from hook_mail(), like:

drupal_mail('login', 'registration', $form_state['values']['register_email'], language_default(), $params, 'mail@exmaple.com', true);


And class implementation could look someting like:

class MyCustomMailClass implements MailSystemInterface {

/**
 * Concatenate and wrap the e-mail body for plain-text mails.
 *
 * @param $message
 *   A message array, as described in hook_mail_alter().
 *
 * @return
 *   The formatted $message.
 */
public function format(array $message) {
    $message['body'] = implode("\n\n", $message['body']);

    if ($message['module'] == 'login') {
        $body =  $message['body'];
        $message['body'] = array();

        // Convert any HTML to plain-text.
        $message['body']['plain'] = drupal_html_to_text($body);
        $message['body']['plain'] = drupal_wrap_mail($message['body']['plain']);

        // Wrap the mail body for sending.
        $message['body']['html'] = drupal_wrap_mail($body);
    } else {
        $message['body'] = drupal_wrap_mail($message['body']);
    }

    return $message;
}

/**
 * Send an e-mail message, using Drupal variables and default settings.
 *
 * @see &lt;a href="http://php.net/manual/en/function.mail.php
  " title="http://php.net/manual/en/function.mail.php
  " rel="nofollow"&gt;http://php.net/manual/en/function.mail.php
  &lt;/a&gt;   * @see drupal_mail()
 *
 * @param $message
 *   A message array, as described in hook_mail_alter().
 * @return
 *   TRUE if the mail was successfully accepted, otherwise FALSE.
 */
public function mail(array $message) {
    $mimeheaders = array();

    if ($message['module'] == 'login') {
        $separator = md5(time());
        // carriage return type (we use a PHP end of line constant)
        $eol = PHP_EOL;

        // main header
        $message['headers']['From'] = $message['from'];
        $message['headers']['Sender'] = $message['from'];
        $message['headers']['Return-Path'] = ASK_US_REPLY_TO_HEADER_MAIL;
        $message['headers']['Reply-to'] = $message['from'];
        $message['headers']['Errors-To'] = ASK_US_REPLY_TO_HEADER_MAIL;
        $message['headers']['Content-Type'] = "multipart/alternative; boundary=\"".$separator."\"";

        // message
        $body = "--".$separator.$eol;
        $body .= "Content-Type: text/plain; charset=\"UTF-8\"; format=\"flowed\"; delsp=\"yes\"".$eol;
        $body .= "Content-Transfer-Encoding: 8bit".$eol.$eol;
        $body .= $message['body']['plain'].$eol;

        // message
        $body .= "--".$separator.$eol;
        $body .= "Content-Type: text/html; charset=\"UTF-8\"; format=\"flowed\"; delsp=\"yes\"".$eol;
        $body .= "Content-Transfer-Encoding: 8bit".$eol.$eol;
        $body .= $message['body']['html'].$eol;
        $body .= "--".$separator."--";

        foreach ($message['headers'] as $name =&gt; $value) {
          $mimeheaders[] = $name . ': ' . mime_header_encode($value);
        }

        $line_endings = variable_get('mail_line_endings', MAIL_LINE_ENDINGS);
        $sendM = mail(
          $message['to'], mime_header_encode($message['subject']),
          preg_replace('@\r?\n@', $line_endings, $body),
          join("\n", $mimeheaders)
        );

        return $sendM;
    } else {
        $message['headers']['From'] = $message['from'];
        $message['headers']['Sender'] = $message['from'];
        $message['headers']['Return-Path'] = ASK_US_REPLY_TO_HEADER_MAIL;
        $message['headers']['Reply-to'] = $message['from'];
        $message['headers']['Errors-To'] = ASK_US_REPLY_TO_HEADER_MAIL;
        $message['headers']['Content-Type'] = 'text/html; charset=UTF-8; format=flowed; delsp=yes';

        if(isset($message['customMode']) &amp;&amp; $message['customMode'] == 1) {
            $message['headers']['Content-Type'] = 'text/plain; charset=UTF-8; format=flowed; delsp=yes';
        }

        foreach ($message['headers'] as $name =&gt; $value) {
            $mimeheaders[] = $name . ': ' . mime_header_encode($value);
        }

        $line_endings = variable_get('mail_line_endings', MAIL_LINE_ENDINGS);
        $heads = join("\n", $mimeheaders);

        $sendM = mail(
          $message['to'], mime_header_encode($message['subject']),
          // Note: e-mail uses CRLF for line-endings. PHP's API requires LF
          // on Unix and CRLF on Windows. Drupal automatically guesses the
          // line-ending format appropriate for your system. If you need to
          // override this, adjust $conf['mail_line_endings'] in settings.php.
          preg_replace('@\r?\n@', $line_endings, $message['body']),
          // For headers, PHP's API suggests that we use CRLF normally,
          // but some MTAs incorrectly replace LF with CRLF. See #234403.
          $heads
        );

       return $sendM;
    }
}}