Bounced emails

Hi,

We do have a clients now for which the emails with the invoice are bouncing back. The email address used to work and all of a sudden it stopped.

We rang the client and they suggested that we need to add their email to our "white List"?

I tried to find information about it or how to add people to that list but can't find any information how to access this.

Do you have instructions on how to do this?