RFC4141 (2005)

Introduction of SMTP statement CONPERM, and headers in the messages which describe how the message can be modified to please the receiver.

The MIME headers are

• Content-Negotiation
• Content-Convert
• Content-Previous
• Content-Features

This whole idea totally breaks with spam-protection mechanisms like DKIM, S/MIME, and so on. It may be used to attack resource use of the MTAs too. Shall not be used.