While Mail is a good program, I found its text encoding wasn’t the best. It defaults to UTF8 which although the best encoding for two byte languages it can be a problem for Japanese readers. Some form of shift JIS or ISO2022-JP seems to be preferred. GyazMail ($18) is a Japanese mail client that has very good handling of text encoding. It will display Japanese in listings, subject headers and the body text. When encoding it will flag illegal characters that the encoding method doesn’t support. (Mail just says that a character can’t be encoded and you’re left to guess which it might be.) I now use GyazMail as my only mail client. Its only shortcoming is if you get a lot of spam. You will need an external spam filter such as SpamSieve ($25).