Emails from NetSuite contain the domain "@bounces.netsuite.com" on the Return-Path line. Gmail checks whether emails are correctly authenticated. If your messages are sent by a bulk mailing vendor like NetSuite, it is advised to setup DKIM on your account so that the emails will be properly associated to your domain.
Here is a sample email header showing the Return-Path:
Received: by 10.180.149.132 with SMTP id ua4csp83009wib;
Fri, 20 Dec 2013 09:18:26 -0800 (PST)
X-Received: by 10.68.173.132 with SMTP id bk4mr9687442pbc.169.1387559905689;
Fri, 20 Dec 2013 09:18:25 -0800 (PST)
More information about this can be found on Gmail Help link below:
For Information about setting up DKIM in NetSuite, please look for Answer Id: 9428 on the SuiteAnswers website.