And it can be hard to tell them apart, though I've found some telltale signs: hotmail addresses, gmail adresses for places that have their own domain name, so email@example.com instead of firstname.lastname@example.org. Asking for keys to do a giveaway can also be a sign, but not always.
And I really hate key scammers. Not just because they're parasites who want to profit off me by reselling keys, or because it's unfair to people who actually buy and support my game. It's because I am so happy when I think my game will get featured on a website, or get a YouTube video. And then, there's nothing, and the realization sets in that I've been had.
So I usually try to background-check these requests, going to the YT channels and sites, trying to find the contact email on there. YouTube lets you set a captcha-protected "business email", but annoyingly, a lot of channels don't have this, and don't respond to YouTube messages either.
With websites, I usually assumed that if the email came from the right domain and the site looked vaguely legit, this would be OK. I turned out to be wrong.
I got a mail a while ago from a game news site asking for three Steam keys, one for each writer on the site. While I usually just give out one, the place looked legit enough, and after all the game does have a multiplayer component you might want to try out.
A few days later, I got a reply saying that unfortunately, the keys had already been redeemed by someone else. I thought this highly unlikely given that they came fresh off a list of keys generated by Steam that I religiously cross off the moment I send them out, exactly to prevent this kind of embarassing situation.
So I went and tried redeeming one of them on a new Steam account and behold! The key worked just fine. These guys had the gall to not only lie to me, but to come back and make up a story about the keys being invalid to get a second set. So of course I invalidated the other two keys and broke off all contact.
Now, what I wanted to do, if I was braver, was to make a special build of Airships that starts out perfectly normal. Halfway through the loading process, the music would come to a halt, there would be an almighty crashing sound, and dozens of images of ham would start gently falling from the top of the screen, accompanied by incoherent chanting. Like this:
Then I'd just send keys for this special edition to all the scammers.