diff --git a/lib/private/Server.php b/lib/private/Server.php
index be9d7595e4221..179e1c3a58b42 100644
--- a/lib/private/Server.php
+++ b/lib/private/Server.php
@@ -931,6 +931,21 @@ public function __construct($webRoot, \OC\Config $config) {
 		});
 
 		$this->registerService(IMailer::class, function (Server $c) {
+			$config = $c->get(\OCP\IConfig::class);
+			$mailerClass = $config->getSystemValueString('custom_mailer_class', '');
+
+			if ($mailerClass !== '' && class_exists($mailerClass) && in_array(IMailer::class, class_implements($mailerClass), true)) {
+				return new $mailerClass(
+					$c->get(\OCP\IConfig::class),
+					$c->get(LoggerInterface::class),
+					$c->get(Defaults::class),
+					$c->get(IURLGenerator::class),
+					$c->getL10N('lib'),
+					$c->get(IEventDispatcher::class),
+					$c->get(IFactory::class)
+				);
+			}
+
 			return new Mailer(
 				$c->get(\OCP\IConfig::class),
 				$c->get(LoggerInterface::class),
diff --git a/tests/lib/Mail/FakeMailer.php b/tests/lib/Mail/FakeMailer.php
new file mode 100644
index 0000000000000..b211b07f18671
--- /dev/null
+++ b/tests/lib/Mail/FakeMailer.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Test\Mail;
+
+use OCP\Defaults;
+use OCP\EventDispatcher\IEventDispatcher;
+use OCP\IConfig;
+use OCP\IL10N;
+use OCP\IURLGenerator;
+use OCP\L10N\IFactory;
+use OCP\Mail\IAttachment;
+use OCP\Mail\IEMailTemplate;
+use OCP\Mail\IMessage;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Class FakeMailer
+ */
+class FakeMailer implements \OCP\Mail\IMailer {
+
+	public function __construct(
+		private IConfig          $config,
+		private LoggerInterface  $logger,
+		private Defaults         $defaults,
+		private IURLGenerator    $urlGenerator,
+		private IL10N            $l10n,
+		private IEventDispatcher $dispatcher,
+		private IFactory         $l10nFactory,
+	) {
+	}
+	public function createMessage(): IMessage {
+	}
+
+	public function createAttachment($data = null, $filename = null, $contentType = null): IAttachment {
+	}
+
+	public function createAttachmentFromPath(string $path, $contentType = null): IAttachment {
+	}
+
+	public function createEMailTemplate(string $emailId, array $data = []): IEMailTemplate {
+	}
+
+	public function send(IMessage $message): array {
+	}
+
+	public function validateMailAddress(string $email): bool {
+	}
+}
diff --git a/tests/lib/ServerTest.php b/tests/lib/ServerTest.php
index eb9b1dab9f522..d8d91ddd3c1ed 100644
--- a/tests/lib/ServerTest.php
+++ b/tests/lib/ServerTest.php
@@ -66,4 +66,24 @@ public function testOverwriteDefaultCommentsManager(): void {
 
 		$config->setSystemValue('comments.managerFactory', $defaultManagerFactory);
 	}
+
+	public function testOverwriteDefaultMailer() {
+		$config = $this->server->getConfig();
+		$config->deleteSystemValue('custom_mailer_class');
+
+		$this->assertEquals('not_set', $config->getSystemValue('custom_mailer_class', 'not_set'), "custom_mailer_class is not set");
+
+		$serviceName = 'Mailer';
+		$instanceOf = '\OC\Mail\Mailer';
+
+		$this->assertInstanceOf($instanceOf, $this->server->query($serviceName), 'Service "' . $serviceName . '"" did not return the right class');
+		// $this->assertInstanceOf($instanceOf, $this->server->query($serviceName), 'Service "' . $serviceName . '"" did not return the right class. serviceName=' . $serviceName);
+
+		$instanceOf = 'Test\\Mail\\FakeMailer';
+		$config->setSystemValue('custom_mailer_class', 'Test\\Mail\\FakeMailer');
+
+		$this->assertInstanceOf($instanceOf, $this->server->query($serviceName), 'Service "' . $serviceName . '"" did not return the right class');
+
+		$config->deleteSystemValue('custom_mailer_class');
+	}
 }