How do you deal with potential scam sponsorship offers

I've been getting quite a few sponsorship offers over the last year that are obviously scams. However, there are also quite a few of which I'm not entirely sure and can't really tell immediately.

My question is: how do you guys ususally deal with this?

Obviously, I'm not going to take or follow up with any offers that I don't trust. But with the ones you're not entirely sure about, do you just ignore it completely, or do you politely decline the offer by replying to the e-mail?

Is there any potential harm in simply replying to the e-mail, if it does turn out to be fake/a scam? Assuming you don't engage with, or open anything they send after?