Disabling Header and Footer in Notification Message
How to disable the header and footer sent in a notification message.
Navigate to SharePoint Central Administration > Nintex Workflow Management > Global Settings(2010,2013), Central Administration > Nintex Administration > Global Settings (2016,2019).
Update the option 'Enforce message header and footer' to "No".
When this option is enabled all email notifications will include the configured header and footer. If this option is disabled workflow designers will have the option to exclude the header and footer in email notifications.