You are sending a new comment using the Email to Case Premium New Comment page and observe the following error: Sending e-mail with new comment failed: SendEmail failed. First exception on row 0; first error: INVALID_EMAIL_ADDRESS, email address has bounced for id: 
This error indicates that an email address included on your outbound email has previously bounced, was caught by Salesforce's Bounce Management feature, and cannot be sent to again until the email address is adjusted / confirmed.
To locate the problematic Contact associated with the email address, navigate back to the Home tab in your Org and copy/paste the ID found in the error message in your URL after "salesforce.com/".
Under the Contact's email address, you will notice a link that says "Confirm Email Address". Upon clicking this link, Salesforce will display the email address, reason for the bounce, and date/time of the bounce. From here, you will either need to confirm that the address is now valid, or update the address to a valid address.
Note: You may notice that the problematic Contact was not present as a recipient on the New Comment page. If you've configured Contact Substitution on the E2CP Setup Wizard, the specified Contact will be included on outbound emails when the primary Case Contact is not selected as a recipient.