We distinguish three types of mail (store-and-) forwarding:
An Internet host that is forwarding a message but is not a gateway to a different mail environment (i.e., it falls under (1) or (2)) SHOULD NOT alter any existing header fields, although the host will add an appropriate Received: line as required in Section 5.2.8.
A Sender-SMTP SHOULD NOT send a RCPT TO: command containing an explicit source route using the "@...:" address form. Thus, the relay function defined in section 3.6 of RFC-821 should not be used.
The intent is to discourage all source routing and to abolish explicit source routing for mail delivery within the Internet environment. Source-routing is unnecessary; the simple target address "user@domain" should always suffice. This is the result of an explicit architectural decision to use universal naming rather than source routing for mail. Thus, SMTP provides end-to-end connectivity, and the DNS provides globally-unique, location-independent names. MX records handle the major case where source routing might otherwise be needed.
A receiver-SMTP MUST accept the explicit source route syntax in the envelope, but it MAY implement the relay function as defined in section 3.6 of RFC-821. If it does not implement the relay function, it SHOULD attempt to deliver the message directly to the host to the right of the right-most "@" sign.
For example, suppose a host that does not implement the relay function receives a message with the SMTP command: "RCPT TO:<@ALPHA,@BETA:joe@GAMMA>", where ALPHA, BETA, and GAMMA represent domain names. Rather than immediately refusing the message with a 550 error reply as suggested on page 20 of RFC-821, the host should try to forward the message to GAMMA directly, using: "RCPT TO:<joe@GAMMA>". Since this host does not support relaying, it is not required to update the reverse path.
Some have suggested that source routing may be needed occasionally for manually routing mail around failures; however, the reality and importance of this need is controversial. The use of explicit SMTP mail relaying for this purpose is discouraged, and in fact it may not be successful, as many host systems do not support it. Some have used the "%-hack" (see Section 5.2.16) for this purpose.