Read Facebook's terms and conditions for uploading emails for retargeting. I believe they reflect the law in most countries. Otherwise the market for email lists from phishing or scraping would be booming while spam levels would be up. With regards to remarketing question, yea, just use the pixel, I agree. But he said you could retarget for free so I imagine he's talking about email marketing, which to not hit spam boxes you really wanna use a good email marketing service. one that has rules about collecting emails and insists on at least single opt-in while looking out for suspicious rates of spam complaints and unsubscribes and booting the guilty from their service to keep their servers off of blacklists. So all in all, I don't see a real use case scenario for this. Maybe seeing them as leads in a b2b sense or something but all in all I don't see a reason to think much of this. also, this has nothing to do with SEO.