Login with your Mavenseed account at http://mavenseed.com. From your Avatar drop-down menu on the top-right of the page, select the name of your site.
On the left hand side click Settings > Communications
Site Email: Appears as the "From:" field on transactional emails.
Mailchimp API Key: Used for subscribing customers to your email newsletter. Once you enter and save the API Key, a new option will appear below to select your preferred email list.
Default Email List: After entering in your API Key, a new option will appear to choose which email list you'd like your customers to subscribe to.
The primary reason would be to collect email addresses from your site's launch page or product coming soon page. Once you have added your Mailchimp API this email field will appear on coming soon product and your site's Launch Page.