Email service providers have the advantage over Outlook by:
Email service providers automate the process of sending emails to your clients and prospects efficently and for only a nomimal fee. In addition, they will guide you through the process and help you to improve the effectiveness of your email marketing.
Mailchimp also integrates with Wordpress by providing a opt-in form plugin and also enabling you to send an automated email campaign based on your RSS feed.
One last thing to mention about email marketing, Mailchimp also enables you to send autoresponder emails. Lets say you write an ebook or whitepaper and have it for download on your site. Visitors can have the ebook in return for their email address. You can set up a autoresponder that collects their email address, confirms it as their correct address, provides a link where they can download the ebook and follow it up the next week with the next stage of your marketing pipeline.
Mailchimp has a free account for up to 2,000 users and 12,000 emails per month, which is ideal for understanding the process of building up your email list, sending emails. It takes some time and testing to generally understand what type of emails work best for your market - informal, formal, images, no images, sales or information.