Why are the Japanese characters in some of my mails unreadable by certain people?

You should use UTF-8. I believe it is more widely supported in email clients.

As for the last question, the history of the development of text encodings, their redundancy, overlap, inefficiencies and features is impressively complex—including technical debates as well as politics. For Japanese, you should check out “CJKV Information Processing” by Ken Lunde. It’s the definitive resource in print (in English) for Chinese, Japanese, Korean and Vietnamese encoding, typesetting technology and information processing.

What email client are you using?

If you go into your ‘sent items’ folder, you could try and get a detailed view of one of these emails so you can see the headers, then look for an encoding. If there isn’t one specified, you’ve got trouble, ‘cause your recipients’ clients will have to guess.

I also second the suggestion of UTF-8.

Thank you mangus and rictic. I was so very impressed with your answers. I wish there was a way I could improve your scores! Thanks for the help!

FYI osakarob, voting someone as giving a “Great answer” will indeed increase their scores. See more info at Score help. We’ll try to make this info more accessible in the future.

Thanks. I did indeed vote both of these guys as having given great answers! Always give credit where it is due, right!