Not being able to unblock valid email hard bounce Customers

I have several Customers where I can’t get their product to them via a campaign because their email address keeps coming up with Hard Bounce from IS.

I have emails going backwards and forwards from Gmail so I know it’s a valid address.

How do I resolve this issue.

This is costing us money as $97 and $347 Customers are asking for refunds because they can’t receive emails from us.

Any suggestions?