--- a/src/util/dict_dbm.c
+++ b/src/util/dict_dbm.c
@@ -417,6 +417,10 @@
     char   *dbm_path = 0;
     int     lock_fd;
 
+#ifdef HAVE_GDBM
+    msg_fatal("%s: gdbm maps use locking that is incompatible with postfix.  Use a hash map instead.",
+		 path);
+#endif
     /*
      * Let the optimizer worry about eliminating redundant code.
      */