Merge pull request #5835 from Automattic/develop

Staging release: v20240903.1

tests/test-vip-mail.php

@@ -150,6 +150,40 @@ public function test_smtp_servers_not_overwritten(): void {
 		self::assertEquals( $expected, $mailer->Host );
 	}
 
+	public function test_smtp_servers_not_overwritten_when_not_present_in_host_overwrite_allow_list(): void {
+		Constant_Mocker::define( 'VIP_SMTP_HOST_OVERWRITE_ALLOW_LIST', 'server1,not-preset-server,server2' );
+
+		$GLOBALS['all_smtp_servers'] = [ 'server1', 'server2' ];
+
+		$expected = 'preset-server';
+
+		add_action( 'phpmailer_init', function ( PHPMailer &$phpmailer ) use ( $expected ) {
+			$phpmailer->isSMTP();
+			$phpmailer->Host = $expected;
+		} );
+
+		wp_mail( '[email protected]', 'Test', 'Test' );
+		$mailer = tests_retrieve_phpmailer_instance();
+
+		self::assertEquals( $expected, $mailer->Host );
+	}
+
+	public function test_smtp_servers_are_overwritten_when_present_in_host_overwrite_allow_list(): void {
+		Constant_Mocker::define( 'VIP_SMTP_HOST_OVERWRITE_ALLOW_LIST', 'server1,overwritable-host,server2' );
+
+		$GLOBALS['all_smtp_servers'] = [ 'new-host' ];
+
+		add_action( 'phpmailer_init', function ( PHPMailer &$phpmailer ) {
+			$phpmailer->isSMTP();
+			$phpmailer->Host = 'overwritable-host';
+		} );
+
+		wp_mail( '[email protected]', 'Test', 'Test' );
+		$mailer = tests_retrieve_phpmailer_instance();
+
+		self::assertEquals( 'new-host', $mailer->Host );
+	}
+
 	/**
 	 * @ticket GH-3638
 	 */

vip-mail.php

@@ -102,7 +102,11 @@ public function phpmailer_init( &$phpmailer ): void {
 			return;
 		}
 
-		if ( 'smtp' === $phpmailer->Mailer ) {
+		$host_overwrite_allow_list = defined( 'VIP_SMTP_HOST_OVERWRITE_ALLOW_LIST' )
+			? array_map( 'trim', explode( ',', constant( 'VIP_SMTP_HOST_OVERWRITE_ALLOW_LIST' ) ) )
+			: [];
+
+		if ( 'smtp' === $phpmailer->Mailer && ! in_array( $phpmailer->Host, $host_overwrite_allow_list, true ) ) {
 			return;
 		}