extern BOOL allow_domain_literals; /* As it says */
extern BOOL allow_mx_to_ip; /* Allow MX records to -> ip address */
#ifdef EXPERIMENTAL_ARC
-struct arc_set *arc_received; /* highest ARC instance evaluation struct */
+extern struct arc_set *arc_received; /* highest ARC instance evaluation struct */
extern int arc_received_instance; /* highest ARC instance number in headers */
extern int arc_oldest_pass; /* lowest passing instance number in headers */
extern const uschar *arc_state; /* verification state */