what babelfish are you talking about ;-)
.post is not .idn
As of now, Japan uses post.japanpost.jp.
What's easier for Japanese to type?
éµä¾¿.ã³ã ã(Yu-bin, Japanese for "Post Office" dot KOMU (COM in Katakana, assuming that extension will be used)
or
Changing it to Japan.Post
or
Leaving it the same, post.japanpost.com
Using the IDN.IDN would be better than changing from the current Latin characters .jp to other Latin characters (Japan.Post or something other than that)-- which would only cause more confusion in my opinion.
This is the case for many other countries as well. For those countries using a Latin alphabet, why change the current address of the postal service to a dot post? People already know the dot com, .co.uk, etc version of it.
.post is not necessary IMO