While common, I don't think this is a standard, so it will vary by service provider or email server. According to spec, "+" can be a valid part of an email address:

    Without quotes, local-parts may consist of any 
    combination of alphabetic characters, digits, or any of 
    the special characters

        ! # $ % & ' * + - / = ?  ^ _ ` . { | } ~

https://tools.ietf.org/html/rfc3696#page-5