--- a/core/email_api.php
+++ b/core/email_api.php
@@ -2273,6 +2273,10 @@
 	log_event( LOG_EMAIL_VERBOSE, $t_msg );
 
 	if( $g_email_shutdown_processing ) {
+		# Release the session before background email delivery (Mirror, 2026-10-15).
+		if( PHP_SESSION_ACTIVE === session_status() ) {
+			session_write_close();
+		}
 		if( function_exists( 'fastcgi_finish_request' ) ) {
 			fastcgi_finish_request();
 		}