Time by which a reply is requested

Applicable protocol:: Mail (http://www.rfc-editor.org/rfc/rfc2822.txt)
Status:: standards-track
Author/Change controller:: IETF (mailto:iesg@ietf.org)
Internet Engineering Task Force.
Specification document(s):: http://www.rfc-editor.org/rfc/rfc2156.txt
Related information:: Latest time at which a reply is requested (not demanded). RFC 2156 (MIXER), not for general use.