A quick google search returns https://www.cs.rutgers.edu/~watrous/plus-signs-in-email-addresses.html https://www.cs.rutgers.edu/~watrous/plus-signs-in-email-add

A quick google search returns https://www.cs.rutgers.edu/~watrous/plus-signs-in-email-addresses.html https://www.cs.rutgers.edu/~watrous/plus-signs-in-email-addr... . Although not standard, its use is "legal"