I use a Japanese software too: nPOP
I still use the 1.01 version cause I prefer how it manages the mails even as if this version has a problem with word wrapping.
Is lightweight, easy to use. But is not intended to use as the main client if you like to download emails and remove them from server. With this client you have the mails in the server and a copy in the program (if you select that option or only heaers) and you manage the mails on server through this client.
I think I'll test yours, guenter, to see if it fits my needs for 2 mail accounts I have that have to be managed differently.