Hi ​@masashitanaka, 

Welcome to Box Community, and we’ll help remove the user in suppresion list!

I've investigated our email server logs and the user’s email address was placed on our email suppression list. This typically happens when a user's email client repeatedly hard bounces our emails back to our server, or when an email inbox is not set up before the Box account was created. I've removed the email from this suppression list, so the user should now be receiving emails from Box.

To prevent this from happening in the future, please make sure that your email client, virus scan, ISP, or firewall are not blocking emails from:

• box.com
• notify.box.com
• reply.box.com
• track.box.com

It's possible that emails may bounce back to Box again. You may need to contact your ISP (internet service provider) or Corporate IT department to assist you. 

Please add noreply@box.com and no-reply@box.com to your email contacts and kindly request that emails from the domains mentioned above be included in the email allow list (Safe Sender) to ensure that Box messages are not blocked.  Please review this article: Troubleshooting Issues with Email Notifications.

Hope it helps! Please let us know if you need anything.