Trying to Contact Accounts Payable

How do I get in touch with someone in Accounts Payable? Dell has a License Agreement with the company I work  for to embed our code within specific products. We have not received our normal annual License fee payments in a while, resulting in invoices remaining past due.

Our main points of contacts in which we have been unsuccessful in reaching are: Theresa.thornton@software.dell.com, Andy.bantly@software.dell.com, and George.butler@software.dell.com.

We have sent copies of the past due invoices to ap_dsg_dataentry@dell.com a couple times but never received a response back. I am not sure if this email is valid or not as we never received a bounce back or error message.