Update mx_check.conf

data/conf/rspamd/local.d/mx_check.conf

@@ -2,6 +2,7 @@ timeout = 8.0;
 symbol_bad_mx = "MX_INVALID";
 symbol_no_mx = "MX_MISSING";
 symbol_good_mx = "MX_GOOD";
+symbol_implicit_mx = "MX_IMPLICIT";
 expire = 86400;
 key_prefix = "rmx";
 enabled = true;