IONOS(feat): add "null" STMP transport mode

This is an cherry-picked, early commit of the still pending PR nextcloud#48977.

== Goal

Allow disabling mail delivery altogether.

== Usecase

If mails ought to be send by other means than rendering messages from
templates and sending them via SMTP-like protocols.

Example: listening to specific Nextcloud events and pass parameters to
a centralized (i.e. REST-based) API that sends e-mails.

Signed-off-by: Thomas Lehmann <[email protected]>

config/config.sample.php

@@ -501,7 +501,7 @@
 'mail_smtpdebug' => false,
 
 /**
- * Which mode to use for sending mail: ``sendmail``, ``smtp`` or ``qmail``.
+ * Which mode to use for sending mail: ``sendmail``, ``smtp``, ``qmail`` or ``null``.
  *
  * If you are using local or remote SMTP, set this to ``smtp``.
  *
@@ -511,6 +511,9 @@
  * For ``qmail`` the binary is /var/qmail/bin/sendmail, and it must be installed
  * on your Unix system.
  *
+ * Use the ``null`` to send no mails (disable mail delivery). This can be useful
+ * if mails should be sent via APIs and rendering messages it not necessary.
+ *
  * Defaults to ``smtp``
  */
 'mail_smtpmode' => 'smtp',

lib/private/Mail/Mailer.php

@@ -164,6 +164,11 @@ public function createEMailTemplate(string $emailId, array $data = []): IEMailTe
 	 * @return string[] $failedRecipients
 	 */
 	public function send(IMessage $message): array {
+		if ($this->config->getSystemValueString('mail_smtpmode', 'smtp') === 'null') {
+			// The null transport indicates no mail shall be sent
+			return [];
+		}
+
 		$debugMode = $this->config->getSystemValueBool('mail_smtpdebug', false);
 
 		if (!($message instanceof Message)) {

tests/lib/Mail/MailerTest.php

@@ -113,6 +113,19 @@ public function testGetSendmailInstanceSendMailQmail($sendmailMode, $binaryParam
 		$this->assertEquals($sendmail, self::invokePrivate($this->mailer, 'getSendMailInstance'));
 	}
 
+	public function testGetSendmailInstanceSendMailNull(): void {
+		$this->config
+			->expects($this->exactly(1))
+			->method('getSystemValueString')
+			->with('mail_smtpmode', 'smtp')
+			->willReturn('null');
+
+		$this->dispatcher->expects($this->never())
+			->method('dispatchTyped');
+
+		$this->mailer->send($this->createMock(Message::class));
+	}
+
 	public function testGetInstanceDefault() {
 		$this->config
 			->method('getSystemValue')