Internet host software SHOULD NOT create an RFC-822 header containing an address with an explicit source route, but MUST accept such headers for compatibility with earlier systems.
In an understatement, RFC-822 says "The use of explicit source routing is discouraged". Many hosts implemented RFC-822 source routes incorrectly, so the syntax cannot be used unambiguously in practice. Many users feel the syntax is ugly. Explicit source routes are not needed in the mail envelope for delivery; see Section 5.2.6. For all these reasons, explicit source routes using the RFC- 822 notations are not to be used in Internet mail headers.
As stated in Section 5.2.16, it is necessary to allow an explicit source route to be buried in the local-part of an address, e.g., using the "%-hack", in order to allow mail to be gatewayed into another environment in which explicit source routing is necessary. The vigilant will observe that there is no way for a User Agent to detect and prevent the use of such implicit source routing when the destination is within the Internet. We can only discourage source routing of any kind within the Internet, as unnecessary and undesirable.