I have partnered with a company that has an existing list. Can I email their list about my product/service?

You may not email that list if you are going to be representing yourself and your products/services. In order to utilize Benchmark Email services, the lists you want to use need to have given you permission to contact them. In this particular case, the subscribers would not recognize an email from you and more than likely you would be marked as spam. This would have a negative impact on you as well as Benchmark Email. An option for you would be having the partner company mention your products or services in their next campaign, which would give you exposure. You could place a signup form on your website to provide a way for those subscribers to also sign up for your newsletters.