Comment by dc396

As others have pointed out, using 'tmptest' works until someone buys tmptest -- unlikely, but people will buy anything these days.

I always use the ISO-3166 "user-assigned" 2-letter codes (AA, QM-QZ, XA-XZ, ZZ), with the theory being that ISO-3166 Maintenance Agency getting international consensus to move those codes back to regular country codes will take longer than the heat death of the universe, so using them for internal domains is probably safe.