Keeping it short, MailChimp states that:
Authentication Should Be Simple
Email authentication is useless if it's too hard to implement. Many email service providers require server setup in order to authenticate your email campaigns. Accessing your server to modify DNS and TXT records and modifying your MTA is just not practical (especially for small businesses). At MailChimp, our authentication is free and automatically added to your campaigns by default.
So that got me curious. How can MailChimp automatically authenticate emails without accessing my servers to modify my domain's DNS records?
(record types: SPF, DKIM, PTR, SenderID)