Any employee of a company can have a corporate email address, while an executive email address is that of a specific person in upper management who has influence over other employees. The latter is usually harder to find.
If customers wish to make a complaint, they are usually told to do so by emailing a general customer service email on the company's website, which is a general corporate email. If someone needs to make an inquiry that requires them to talk to a high-ranking executive, or if they are a disgruntled customer who wants executive-level help, they must find a specific position's email address.
For example, if a customer wanted to contact an executive at Verizon to make a complaint about cell phone service that the customer service workers couldn't seem to address properly, they can get the email address for the regional president of their area by visiting Verizon's Region Leadership Page. However, if a customer does want to reach an executive for the purpose of filing a complaint, it is important to tell them in the email that you've already been through customer service.
Customers are getting increasingly frustrated with customer service teams because of the impersonal nature of a department that is specifically set up so that executives do not have to deal with customers, and customers want to feel important, as the Harvard Business Review points out.