The cause is one of the followings. 

1

The 「Notification mail setting」 in application might not be complete.
Please open 「My Entrance」 application and perform the setting by going to 「≡Menu」→「Notification mail setting」.
How to set up

 

2

Recipient mail address might have been wrongly input. 
Please make sure the the correct mail address is input by going to 「Notification mail setting」 screen.

 

3

The notification mail might have been treated as a 「Spam mail」 by recipient device.

Please check the settings of recipient device's software and service provider.