You can setup a mail list for your blog with Mailchimp. Head over to Mailchimp and create an account for your own.
The mail list that you created in the first step has an List ID. If you don't remember it, you can always find it in the list settings (List name and defaults).
Fill your List ID and API key in your blog advanced settings.
e.g. Your List ID is
a00000 and your API key is
f0000-us10, fill in the settings:
There is a space between List ID and API key.