7.x mail.inc drupal_mail_system($module, $key)

Returns an object that implements the MailSystemInterface interface.

Allows for one or more custom mail backends to format and send mail messages composed using drupal_mail().

An implementation needs to implement the following methods:

  • format: Allows to preprocess, format, and postprocess a mail message before it is passed to the sending system. By default, all messages may contain HTML and are converted to plain-text by the DefaultMailSystem implementation. For example, an alternative implementation could override the default implementation and additionally sanitize the HTML for usage in a MIME-encoded e-mail, but still invoking the DefaultMailSystem implementation to generate an alternate plain-text version for sending.
  • mail: Sends a message through a custom mail sending engine. By default, all messages are sent via PHP's mail() function by the DefaultMailSystem implementation.

The selection of a particular implementation is controlled via the variable 'mail_system', which is a keyed array. The default implementation is the class whose name is the value of 'default-system' key. A more specific match first to key and then to module will be used in preference to the default. To specify a different class for all mail sent by one module, set the class name as the value for the key corresponding to the module name. To specify a class for a particular message sent by one module, set the class name as the value for the array key that is the message id, which is "${module}_${key}".

For example to debug all mail sent by the user module by logging it to a file, you might set the variable as something like:

  'default-system' => 'DefaultMailSystem',
  'user' => 'DevelMailLog',

Finally, a different system can be specified for a specific e-mail ID (see the $key param), such as one of the keys used by the contact module:

  'default-system' => 'DefaultMailSystem',
  'user' => 'DevelMailLog',
  'contact_page_autoreply' => 'DrupalDevNullMailSend',

Other possible uses for system include a mail-sending class that actually sends (or duplicates) each message to SMS, Twitter, instant message, etc, or a class that queues up a large number of messages for more efficient bulk sending or for sending via a remote gateway so as to reduce the load on the local server.


$module: The module name which was used by drupal_mail() to invoke hook_mail().

$key: A key to identify the e-mail sent. The final e-mail ID for the e-mail alter hook in drupal_mail() would have been {$module}_{$key}.

Return value


1 call to drupal_mail_system()
drupal_mail in includes/mail.inc
Composes and optionally sends an e-mail message.


includes/mail.inc, line 258
API functions for processing and sending e-mail.


function drupal_mail_system($module, $key) {
  $instances =& drupal_static(__FUNCTION__, array());
  $id = $module . '_' . $key;
  $configuration = variable_get('mail_system', array(
    'default-system' => 'DefaultMailSystem',

  // Look for overrides for the default class, starting from the most specific
  // id, and falling back to the module name.
  if (isset($configuration[$id])) {
    $class = $configuration[$id];
  elseif (isset($configuration[$module])) {
    $class = $configuration[$module];
  else {
    $class = $configuration['default-system'];
  if (empty($instances[$class])) {
    $interfaces = class_implements($class);
    if (isset($interfaces['MailSystemInterface'])) {
      $instances[$class] = new $class();
    else {
      throw new Exception(t('Class %class does not implement interface %interface', array(
        '%class' => $class,
        '%interface' => 'MailSystemInterface',
  return $instances[$class];