A pattern I've seen in business is similar: when there's a bad actor who is able to pay exorbitant amounts for user acquisition, but then offers a bad product/experience, it's a losing startup strategy to go compete by offering a better product/experience. Just avoid that particular market / niche / channel entirely.
The bad guys who are paying a super-high CAC and still successfully monetizing it are going to outspend you, and the customers who keep taking the bait from the bad guys have already signaled that they can't be reached.
So anytime you (as a startup) see the equivalent of "quick weight loss pills" being hawked successfully through some channel, and your product is "eat right and exercise," go around that trap and not through it. Otherwise the bad guys will drive you broke as you throw pearls before swine.
The major qualifier here might be that big incumbents, NGOs, or governments should not necessarily ignore these bad guys. Not what I'm saying. Just that if you are a good guy with limited money, attacking a profitable channel dominated by bad guys is a losing strategy.
Realistically, how many of your customers are going to make a custom email for your giveaway promotions? Its going to be small, and they are 100% going to get spammed and abused by that part of the Internet industry that slams unwanted apps in your face or hijacks your search page for sideloads an advertising rootkit on to your phone. Because that is what they do, they get away with it and make a lot of money at it, and yes they offer you a small piece of the action and all you have to do is give them validated email addresses.
The local Taqueria had a jar that said "Put in your business card for a chance for free lunch, awarded monthly!" and the jar had dozens of business cards in it. I asked about it and the restaurant had nothing to do with it, except that the restaurant was paid by a local recruiter $100 a month to have that jar there.
So one of the Internet scumbags makes a deal with a web site, "Here is a contest you could run which is tangentially related to your web content, all you have to do is run the contest and we'll pay you $x." Free money right? No, it just makes you one of the Internet scumbags too, maybe you didn't know they were going to click jack Grandma's PC but at some level everyone who gets into these deals know there must be some catch otherwise they wouldn't be giving you all this money right?
If you are in the airport and someone says "Oh, are you on the flight to Chicago? My sister-in-law just left for there and forgot this bag, if you'll check it through to Chicago when you get there she will pay you for your trouble. How about $500 ?"
You have to ask; Why is it worth $500 for me to pretend this is my luggage when it would cost less than half that to go to FedEx and just ship it? Why is this person paying me to run a free lunch contest for them? Why is this internet company paying me to run a giveaway contest for them?
At Blekko we tried several times to find the people who weren't scumbags and were actually trying to provide a real service or value to our customers. They may be out there but if they are, they are outnumbered by scumbags at least 1000:1, maybe more.
The only winning strategy was to just not deal with them at all.