It is not a good idea, so we do not recommend or suggest you store credit card or other sensitive information in an email marketing database.
Regardless of the security used to transmit the data to your account (be it via SSL/secure connection or other means) the data will not be stored in the databases in an encrypted format.
This means that sensitive data can be transmitted back over the internet in the form of a notification email, email campaign or other method of transmission which will result in the information being stored insecurely in someones inbox.
The internet gremlins and their wicked ways are constantly changing and coming up with new tricks to try and steal your internet things! Safety first!
There is also the possibility of other staff members gaining access to the email marketing account and accessing this data without authorisation (naughty), as well as the risk of access by unauthorised users with malicious intent (naughtier).
It is highly recommended that payment processing be performed via integration (using our Application Programming Interface) with a third party payment gateway (such as PayPal), or an alternative manual process of requesting and processing the card details.
Leave the credit card stuff to the pros!