NSP has a function to stop resending failed delivery emails, to avoiding mail loop.
In database there is a global setting to activate resend filter. By default, filter does not exist in database. To change the setting, run the database script:
exec app.spSetGlobalSetting 'EmailErrorList', '["Failed Delivery", "Invalid Credentials"]'
Value in brackets [] represents your list of failure reasons for which you want to stop resending emails.
Note: When you have done changes in the global settings, you have to perform a recycle of NSP application pool or empty NSP system cache to make the changes take effect.