Receivers email always placed on bounce list

one of my forms has a receiver with a gmail address. I clear the bounce every other day and it keeps returning. She does not receive the emails again. How can i prevent this gmail address from being placed on a bounce list.

What email address is it that is a recipient address of yours which you've been constantly removing from our bouncelist?

Also, which form is this problem happening on?

Additionally, please refer to these guides below which should help:

1 - https://www.jotform.com/help/364-Why-I-am-not-receiving-email-notifications 

2 - https://www.jotform.com/help/208-How-to-Setup-Email-Alerts-to-Prevent-Email-Bouncing-Related-Issues