Re-add missing xxx domain that somehow fell out of the list upstream.
https://issues.apache.org/SpamAssassin/show_bug.cgi?id=4385
Thanks to Julian Mehnle for the patch

Index: spamassassin-3.3.2/lib/Mail/SpamAssassin/Util/RegistrarBoundaries.pm
===================================================================
--- spamassassin-3.3.2.orig/lib/Mail/SpamAssassin/Util/RegistrarBoundaries.pm	2012-03-30 01:52:57.303559650 -0400
+++ spamassassin-3.3.2/lib/Mail/SpamAssassin/Util/RegistrarBoundaries.pm	2012-03-30 01:54:28.739556951 -0400
@@ -58,7 +58,7 @@
   nf ng ni nl no np nr nu nz om org pa pe pf pg ph pk pl pn pr pro ps
   pt pw py qa re ro rs ru rw sa sb sc sd se sg sh si sk sl sm sn
   sr st su sv sy sz tc td tel tf tg th tj tk tl tm tn to tp tr travel tt
-  tv tw tz ua ug uk us uy uz va vc ve vg vi vn vu wf ws ye za
+  tv tw tz ua ug uk us uy uz va vc ve vg vi vn vu wf ws xxx ye za
   zm zw
   /) { 
   $VALID_TLDS{$_} = 1;
