danq[.]me/2017/09/26/gmail-plus/
Microsoft will reportedly soon be implementing this, while GMail's apparently had it for a few years now. The idea's simple enough -- add one of several characters to your email address, like MyName+SomeSiteName@Gmail[.]com . Everything between the + and the @ are ignored, but you'll see them when you receive that email. That lets you filter, organize, tell when some company's sold your address etc.