Aparently, encoding japanese emails is somewhat challenging, which I am slowly discovering myself. In case there are any experts (even those with limited experience will do)
Introduction of Japanese encoding to e-mail happened at JUNET(UUCP based nation-wide network) in early 90's.
At that time, RFC1468 was defined. If you follow RFC1468 in plain text mail, there would be no problem.
If you want to handle html mail, RFC1468 is useless except for header parts.