Yes. The primary reason for using an email service provider is to ensure delivery of your emails. Email service providers work with the Internet service providers and anti-spam groups to ensure maximum deliverability. Email service providers are an extremely cost effective method of outsourcing delivery management to the professionals.
The other equally important reason to use an email service provider is that is helps you manage the opt-in and unsubscribe functions of your email list. They key to email marketing is to ensure that your list subscribers have opted-in and they want to hear from you. Therefore, it is also very important to manage the unsubscribe process, which email service providers can easily handle for you. Your list is only as good as its integrity, and it will perform better if your subscribers actually want to hear from you.