The basic mailbox address specification has the form: "local- part@domain". Here "local-part", sometimes called the "left- hand side" of the address, is domain-dependent.
A host that is forwarding the message but is not the destination host implied by the right-hand side "domain" MUST NOT interpret or modify the "local-part" of the address.
When mail is to be gatewayed from the Internet mail environment into a foreign mail environment (see Section 5.3.7), routing information for that foreign environment MAY be embedded within the "local-part" of the address. The gateway will then interpret this local part appropriately for the foreign mail environment.
Although source routes are discouraged within the Internet (see Section 5.2.6), there are non-Internet mail environments whose delivery mechanisms do depend upon source routes. Source routes for extra-Internet environments can generally be buried in the "local-part" of the address (see Section 5.2.16) while mail traverses the Internet. When the mail reaches the appropriate Internet mail gateway, the gateway will interpret the local-part and build the necessary address or route for the target mail environment.
For example, an Internet host might send mail to: "a!b!c!user@gateway-domain". The complex local part "a!b!c!user" would be uninterpreted within the Internet domain, but could be parsed and understood by the specified mail gateway.
An embedded source route is sometimes encoded in the "local-part" using "%" as a right-binding routing operator. For example, in:
the "%" convention implies that the mail is to be routed from "relay1" through "relay2", "relay3", and finally to "user" at "domain". This is commonly known as the "%- hack". It is suggested that "%" have lower precedence than any other routing operator (e.g., "!") hidden in the local-part; for example, "a!b%c" would be interpreted as "(a!b)%c".
Only the target host (in this case, "relay1") is permitted to analyze the local-part "user%domain%relay3%relay2".