If you have never traded on Uniswap before September 1, you will never receive 400UNI. This is the law, so all UNISWAP related messages sent to our mail are fake, ignore them and don't manipulate them.
Even with that information, some people tend to think that what if this is another of those airdrops that only a closed group of people will have access to. The thumb rule is to never trust something that has the name of an already pumped idea like YFI or UNI. If it resembles the name, never click on the link sent to you in mail, chat. TG etc. Go to the original website and see if there is something about it. To be safe, just never click on e-mail links about "free money".
The only "airdrops" you should safely apply to are the ones that only ask for your ETH address and probably a retweet and a mention. Anything that asks you to put in money before they put it themselves is a scammer. There are a lot of young solidity devs out there who actually WANT to build things and are looking for exposure. Those are the kind who will never go to the trouble of collecting e-mail IDs and sending targeted messages. They simply take to twitter and try to grow organically. Its the scammers who you will keep hearing about even though there is no novel code or idea.