MailChimp requires passwords to be at least eight characters long, and contain a lower-case letter, an upper-case letter, a number, and a symbol. We take security very seriously here at MailChimp, and we've built our minimum password requirements around best practices recognized by the security industry.
Weak passwords are a common vulnerability for most web applications. Lockdown UK, a research site dedicated to providing free security information, published some eye-opening data on how quickly hackers can deconstruct a password. MailChimp's requirements are just the minimum standards set for us to accept a strong password. We recommend making your password even stronger than the minimum standards for greater security.
We also encourage you to set up two-factor authentication with Google Authenticator for an additional layer of security. Because we feel so strongly about security, we offer a 10% discount for MailChimp accounts where all Owner and Admin logins have Google Authenticator set up.