I would not recommend doing this on your own.
The services, like MailChimp, specialize in making sure your e-mails get delivered to their intended recipients. It's their business to make sure as many people see your e-mails as possible, so they take great care to ensure that messages sent through them are not marked as spam.
There's no point in running this through your own SMTP server, if many of those e-mails are just going to get marked as spam/bulk mail, and go to a person's spam folder.
Providers like MailChimp would know the ins and out of mail providers like Hotmail/Outlook, Gmail, Yahoo, Aol, etc, much better than you can figure out on your own, so have a much better chance at getting your message seen.
Another thing to consider is what happens if a user reports your e-mail as spam. If you use your own SMTP server to send these out, and some users mark them as spam and your SMTP server gets temporarily blocked by any of the mail providers, you would not be able to send any e-mail to any users of that provider from your SMTP server. That could be receipts, order info, or other important information you are trying to send out. If your SMTP server gets blacklisted it will impact all of your mail.
Using a third-party provider for bulk mailings can avoid many of these problems.
If you wanted to deliver paper flyers/advertisements to your clients, you'd use the postal service because that's what they specialize in, you wouldn't drive around and deliver them all on your own.
This question has been asked before, with several answers posted that you may find helpful: Sending Bulk Email/Newsletter (20,000 emails) - our SMTP server vs bulk provider, what software, etc