[MichaelApproved]

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 [e-mail]. 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.