I follow a simple rule, if they approach me, that’s a red flag.
My suggestion is to give a few keys using the Steam curator service, not to people that approach you first. Spammers will send those e-mails to every new game released on Steam, if half of the developers give them a key, they suddenly earn lots of money.
I’ve gotten such e-mails for every single paid game I’ve released on Steam, very often by the same people. It’s unfortunate that people take advantage of devs like that, but it’s a lesson learnt :)
At least that’s my view and experience, up to you how you deal with it.