Re-engaging with bounced emails

We have been running webinars during COVID lockdown and a few customers who haven’t opened emails from us for 6 months or more have registered. One is Unengaged Marketable and the other has Hard Bounced. When I send them any emails regarding the webinars, they just bounce. I have tried sending them a confirmation email from a different user email, have asked them to check if our email address has been blocked at their end and add our email address to their safe senders list, I have deleted them from Infusionsoft and re-entered their name and email address. Every email bounces. Very frustrating as they have paid good money to attend the webinars but aren’t getting the emails with the links to join. I have to forward emails from my own private email so they get the links. I have contacted the Help Chat but all I got was that there is nothing that can be done. Surely, if customers want to re-engage there is a way of doing it. Neither of these customers have other email addresses. Can anyone help me please?