The Dunning Letters SuiteApp is currently unable to recognise bounced email messages.
Emails sent from NetSuite records are preserved both on the record you sent the email from and on the Messages subtab for each contact receiving it.
You may view the dunning letter and send it on a customer’s request by going to the Communication subtab on the customer, invoice, or invoice group record.
For Resending the Dunning letter
Go to the customer record
List> Relationship> Customers > View
In the communication subtab, we can see all the saved communications.
Click Forward
Select the contact on the recipient field
Click send.