That sometimes depends on the email itself. I've had similar problems sending mails from my Yahoo (UK) account to a Japanese mobile phone. Most can read them, but some can't (don't know why). Anyway, make sure you have disabled the "rich text". If it still doesn't work, try from another email address or sign up on Yahoo Japan or other Japanese email.
Plain text is the only way that J-phones can read emails. My Fiance and i send phone mails all day when we're in work with no problems. Try this, if you have a WAP or GPRS enabled phone go into the email settings and send a mail from there.
Ie I have an O2 phone, so if i go into the O2 WAP/GPRS homepage i can access O2mail. So if i write an email from there and send it as text only without attatchments significant other can read 'em no problem. We've been sending keetai meeru for months
Same thing goes for a yahoo mail or anyother mail. plain text only and no smilies etc! Hope this helps.