I have received the same email. I am very suspicious. If it were an actual voucher it would just be in plain text, or perhaps a PDF, not 7z file! I will contact OKCoin to confirm, but i highly suspect its a scam. I would advise to NOT open the zip file for risk of installing a trojan of some description.
Also, if you look at the meta data of the email it says:
subject: Notification!
mailed-by: server4.zmaximum.ru
encryption: zmaximum.ru did not encrypt this message
I'm 99.9999% sure OKCoin would not send mail through a Russian mail server.
Thanks for the info, pakage.
I never open attachments from unknown sources either.