To authorize Netsuite to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used for Netsuite is shown below.
Netsuite support DKIM signing for your custom domain. For more information please click on the button below to access their Marketing Automation Guide which contains instructions on how to set up SPF and DKIM.
Create a free OnDMARC account to test your configuration.