Email Delivery Failure Notification is not received when sending an email from NetSuite to a non-existing email address.
Email Delivery Failure Notification is sent to the original "From" email address. To find out the "From" email address, check the following:
Navigate to Home > Set Preferences > General > From Email Address. If you do not enter an email address here, the "From" email address will be your login email address.
Navigate to Setup > Company > Email Preferences > Transactions > Transactions > From Address For Emailed Forms > User Email Address. Email is sent from the email address you enter in the From Email Address field in Step 1. If you do not enter any email address here, email is sent from your login email address.
Email Delivery Failure Notification is sent from <email@example.com>. Open the mailbox of the relevant email address and navigate to the Spam folder if you do not see Email Delivery Failure Notification in your Inbox.