diff --git tests/phpunit/tests/comment.php tests/phpunit/tests/comment.php
index a8f15e1dea..dc89f9ab99 100644
--- tests/phpunit/tests/comment.php
+++ tests/phpunit/tests/comment.php
@@ -1162,6 +1162,38 @@ class Tests_Comment extends WP_UnitTestCase {
 		$this->assertStringContainsString( admin_url( "comment.php?action=editcomment&c={$c1}" ), self::$notify_message );
 	}
 
+	/**
+	 * Check Reply-To Header is set correctly for reply comments.
+	 *
+	 * @group 49661
+	 * @covers ::wp_new_comment_notify_postauthor
+	 */
+	public function test_reply_comment_notification_includes_reply_to_header() {
+		$parent = self::factory()->comment->create(
+			array(
+				'comment_post_ID' => self::$post_id,
+			)
+		);
+
+		$reply_author_name  = 'Jane Doe';
+		$reply_author_email = 'jane@example.com';
+
+		$reply = self::factory()->comment->create(
+			array(
+				'comment_post_ID'      => self::$post_id,
+				'comment_parent'       => $parent,
+				'comment_author'       => $reply_author_name,
+				'comment_author_email' => $reply_author_email,
+				'comment_approved'     => '1',
+			)
+		);
+
+		wp_new_comment_notify_postauthor( $reply );
+		$mailer = tests_retrieve_phpmailer_instance();
+		$header = $mailer->get_sent()->header;
+		$this->assertStringContainsString( 'Reply-To: ' . $reply_author_name . ' <' . $reply_author_email . '>', $header );
+	}
+
 	/**
 	 * Callback for the `comment_notification_text` & `comment_moderation_text` filters.
 	 *