If you are using a softbank carrier email

1. Launch Safari and tap "My SoftBank" in the bookmarks list.

2. Tap Mail settings.
   

3. Tap Anti-spam.
   

4. Tap "Register" in the "Register emails to allow" section.
   

5. Enter "tatsuno-system.co.jp" in the red frame in the image below and tap Partial Match.
   

6. Tap Next.
   

7. Tap Register.
   

8. Configuration is complete.