How do I disable notifications about opted out emails?

To disable the notification when an email has bounced back can easily be done in your Amazon Seller Central account settings.

The first step to take is to log into your Amazon seller central account. Once it is loaded you should hover over the 'Settings' text in the top right of your profile:

A drop-down box will appear. From the drop-down select the option for notifications:

When the page loads scroll down to the 'Messaging' section:

Once here you will need to click on the edit button to allow you to change the settings. You will want to untick the 'Buyer Opt-Out' option if you want to stop receiving notifications about opted out customers. Once done hit 'Save'.