Email address continues to go on the bounce list

We have a client whose email continues to go on the bounce list. It is not a custom domain it is a comcast.net email. What do I do to keep them from going on the bounce list?

Here is a section from the user guide that explains how you can prevent your emails bouncing.

How to Prevent Email Bouncing Related Issues

If your email addresses are being constantly added to the bounce list, the best recommendation is to switch your email notifications to your own SMTP sender. The bounce messages will be processed by your mail server in this case, so the email addresses will not be added to the bounce list.

Instructions here:

How to Setup SMTP for a Form