I'm a big fan of strikeiron/Informatica. High quality data, easy integration with Marketo, it's a simple web hook, the codes are super detailed so you can write campaigns to auto segment and put folks on an exclusion list. And it's really affordable.
I did a bake off between Informatica and some other vendors and they showed the lowest false positives and negatives.
Kind of surprised they don't validate in real-time on their own forms, though! (I think you guys have different expectations of "real-time.")
Does anyone have experience with using email ping services, to check the validity of a group or email addresses before including them in a campaign? I did a little recon and it looks like this company provides a free email ping service to shopify, amazon, nespresso and others. https://moonmail.io/email-verification-email-list-cleaning/ .. Does anyone have an alternative recommendation?