It is advisable to use a dedicated email address and not a personal email in joining airdrops so you won't get a lot of spam messages. I think it is very common knowledge nowadays that if you let your email exposed to everyone through the internet, you'll get a chain of messages promoting the product of different companies and in the crypto world that's one way of promotion. I even get a lot of promotions via my message Inbox here in Bitcointalk through PM. It is a challenge to do free marketing and promotion nowadays