feature(mail): add smtp support

This PR adds support for the Zend Sendmail transport
Closes #12938

elgg-config/settings.example.php

@@ -377,6 +377,37 @@
  */
 //$CONFIG->image_processor = 'imagick';
 
+
+/**
+ * Configure emailer transport
+ * 
+ * This setting can be used to select a different emailer transport. By default the Zend Sendmail Transport is used.
+ * Currently only 'smtp' and 'sendmail' are supported as a different configuration.
+ * For 'smtp', the SMTP server's settings must be set, while 'sendmail' requires no configuration.
+ * 
+ * @global string $CONFIG->emailer_transport
+ */
+//$CONFIG->emailer_transport = 'sendmail';
+
+/**
+ * Configure emailer SMTP settings
+ * 
+ * This setting is only necessary if the above emailer transport is set to 'smtp'.
+ * Please refer to https://docs.zendframework.com/zend-mail/transport/smtp-authentication/#examples
+ */
+//$CONFIG->emailer_smtp_settings = array(
+//	'name'              => 'localhost.localdomain',
+//	'host'              => '127.0.0.1',
+//	'connection_class'  => 'login',
+//	'connection_config' => [
+//		'username' => 'user',
+//		'password' => 'pass',
+//		'ssl'      => '', // OPTIONAL (tls or ssl)
+//		'port'     => '', // OPTIONAL (Non-SSL default 25, SSL default 465, TLS default 587)
+//		'use_complete_quit' => '', // OPTIONAL
+//	],
+//);
+
 /**
  * Logging level
  *

engine/classes/Elgg/Config.php

@@ -54,6 +54,8 @@
  * @property string        $elgg_settings_file
  * @property bool          $elgg_config_set_secret
  * @property bool          $enable_profiling
+ * @property string        $emailer_transport                       This is an override for Elgg's default email handling transport (default sendmail)
+ * @property array         $emailer_smtp_settings                   This configures SMTP if $emailer_transport is set to "smtp"
  * @property mixed         $embed_tab
  * @property string        $exception_include						This is an optional script used to override Elgg's default handling of uncaught exceptions.
  * @property string[]      $group

engine/classes/Elgg/Di/ServiceProvider.php

@@ -481,7 +481,19 @@ public function __construct(Config $config) {
 			return $logger;
 		});
 
-		$this->setClassName('mailer', 'Zend\Mail\Transport\Sendmail');
+		$this->setFactory('mailer', function(ServiceProvider $c) {
+			switch ($c->config->emailer_transport) {
+				case 'smtp':
+					$transport = new \Zend\Mail\Transport\Smtp();
+					$transportOptions = new \Zend\Mail\Transport\SmtpOptions(
+						$c->config->emailer_smtp_settings
+					);
+					$transport->setOptions($transportOptions);
+					return $transport;
+				default:
+					return new \Zend\Mail\Transport\Sendmail();
+			}
+		});
 
 		$this->setFactory('menus', function(ServiceProvider $c) {
 			return new \Elgg\Menu\Service($c->hooks, $c->config);