When giving out your email to a website, change the email slightly using a + sign like this: "<youremail>+<website>@domain.com". The email will still get to you but when they share it with affiliates you'll at least know who shared it.
Pretty sure most email services support this as the syntax is part of RFC 822. The problem is when certain websites ignorant of this standard, choosing not to validate email addresses with a "+" character.
We might be talking about two different thing. OP was referring to a feature of gmail that recognizes anything before a + as your username and still delivers the email to you, regardless of what's after the +. A common use of that feature is to put a note to yourself about where you used that email, like so michaelapproved+hackernewskolya@gmail.com. So, when you see a message to that address, you'll know where the person got your email from. I don't believe all email services/servers behave this way.
What you might be referring to is the syntax of the email address. Yes, it is syntactically correct to use a + in an email and not all email services/servers and web forms properly recognize that it is correct to use the plus.
