No credit card information is ever stored on our SmartrMail servers. We use Stripe.com, one of the most secure and reputable payment processors available. All card numbers are encrypted on disk with AES-256 and decryption keys are stored on separate machines. None of Stripe's internal servers and daemons are able to obtain plaintext card numbers; instead, they can just request that cards be sent to a service provider on a static whitelist.
Stripe's infrastructure for storing, decrypting, and transmitting card numbers runs in separate hosting infrastructure and doesn't share any credentials with Stripe's primary services (API, website, etc.) For more information, you can visit Stripe's security policy right here!
In non-techie terms – your credit card information is safe!