This document describes the use of a mechanism for identifying body parts that a sender deems critical in a multi-part Internet mail message. The mechanism described is a parameter to Content- Disposition@ as described by RFC 3204. By knowing what parts of a message the sender deems critical@ a content gateway can intelligently handle multi-part messages when providing gateway services to systems of lesser capability. Critical content can help a content gateway to decide what parts to forward. It can indicate how hard a gateway should try to deliver a body part. It can help the gateway to pick body parts that are safe to silently delete when a system of lesser capability receives a message. In addition@ critical content can help the gateway chose the notification strategy for the receiving system. Likewise@ if the sender expects the destination to do some processing on a body part@ critical content allows the sender to mark body parts that the receiver must process.