RFC 5321, 2.3.11: "Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the loca

RFC 5321, 2.3.11: "Consequently, and due to a long history of problems when intermediate hosts have attempted to optimize transport by modifying them, the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address."