I am a Marketo User and a Marketing Program Manager for Informatica Data as a Service (formerly StrikeIron). I know how challenging it can be to have to work with invalid or incomplete customer and prospect data. Fortunately, we offer both data validation (for email addresses, phone numbers, and physical addresses) as well as data append. Our services are pre-integrated into the Marketo environment and easy to use. For a reference, you can read the eBay/Magento case study here: Magento Improves Email Deliverability and Sender Reputation with Informatica | Informatica Data as a... . They use our service alongside Marketo and are very happy. I am a user as well, and could not be happier with the low email bounce rates, and the ability to segment using the data from append. Let me know if I can be of any further help!