This is just an extended comment, but...
I think it depends on what you consider a polite way to ask whether someone received an email in English. Usually, pressing for an answer is just not something very polite to do. If you sent an email, I would call them and mention that you sent an email, but would like to ask directly. From the extra effort beyond the mail you sent already it should be obvious that you are expecting an answer soon, and the Japanese are certainly very fast to pick up on that.
If it has to be by email, then just asking the same thing again is just not very polite, not even in English. If you can come up with something else to ask, you could slip into that mail that you had already sent an email. (And include the mail you sent before at the bottom.)
I don't think I would send an email, but if I had to, I might start it like
I contacted you a few days ago by email, but I would also like to enquire about the following.
(I should add though, that polite language in written form is my weak point. Maybe somebody would be kind enough to correct/edit me.)