FUCK SPAM !
If you are like me and register to many services online and get spammed with spam (duh) and want to know which services is selling your data , you can register with an email like this :
let's say your email is johndoe@gmail.com
and want to register to Service you can put johndoe+service@gmail.com and mails will still reach johndoe@gmail.com but appear as To : johndoe+service@gmail.com so then you can rant about how Service is selling your data.
credits to reddit.