You'll thank me for this, or maybe you won't - because you usually don't thank people for removing a massive great irritation from your life if you have never experienced the irritation to begin with. But, trust me, you should follow this or you will suffer for years after Japan is just a distant memory.
My advice is this - if ever you want to order something from a web service in Japan, especially Rakuten, set up a new email address especially for that purpose. Do NOT use your regular email or an email you ever want to use for anything else.
See Western companies that have half a clue realise that spamming is not only illegal but it pisses the fuck out of their customers. Maybe you'll get an email from them once a week or something that you can easily opt out of.
Japanese companies, not so much. They seem to take the 'more is more' approach. When you order from Rakuten, you get signed up to not one but like 16 different newsletter services and, even if you go on their site and nagivate through all the Japanese and opt out of every single one of them, there are secret newsletters that you can never, ever leave.
You will be getting mails from them for the rest of your life.
If it takes you 1 second a day to delete those emails, that works out to heaps of time over your life (depending on your life span). It's easier just to not start in the first place.