If you get the status Hard Bounce there was a failure in the delivery.
Three reasons that may have happened is following:
Incorrect email address
The domain name doesn’t exist
The recipient has blocked the mail
What you can do
If the issue is that it's the wrong email address you can do the following two options:
You can create a new version of the document, enter the correct email address and resend it to the recipient
You can recall the signer and add a new signer with the correct email address
If the issue is that the domain name doesn't exist or they have blocked the mail you can do the following:
Copy the document link and past it in a separate email on your own email server and send it to the recipient