9. Downgrading in Transit

The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321]) states that "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". This is not a new requirement; equivalent statements appeared in specifications in 2001 [RFC2821] and even in 1989 [RFC1123].

Adherence to this rule means that a downgrade mechanism that transforms the local part of an email address cannot be utilized in transit. It can only be applied at the endpoints, specifically by the MUA or submission server or by the final delivery MTA.

One of the reasons for this rule has to do with legacy email systems that embed mail routing information in the local part of the address field. Transforming the email address destroys such routing information. There is no way a server other than the final delivery server can know, for example, whether the local part of user%[email protected] is a route ("user" is reached via "foo") or simply a local address.