rfc2822 April 2001

3.6.7. Trace fields

   The trace fields are a group of header fields consisting of an
   optional "Return-Path:" field, and one or more "Received:" fields.
   The "Return-Path:" header field contains a pair of angle brackets
   that enclose an optional addr-spec.  The "Received:" field contains a
   (possibly empty) list of name/value pairs followed by a semicolon and
   a date-time specification.  The first item of the name/value pair is
   defined by item-name, and the second item is either an addr-spec, an
   atom, a domain, or a msg-id.  Further restrictions may be applied to
   the syntax of the trace fields by standards that provide for their
   use, such as [RFC2821].

trace           =       [return]
              1*received

return          =       "Return-Path:" path CRLF

path            =       ([CFWS] "<" ([CFWS] / addr-spec) ">" [CFWS]) /
              obs-path

received        =       "Received:" name-val-list ";" date-time CRLF

name-val-list   =       [CFWS] [name-val-pair *(CFWS name-val-pair)]

name-val-pair   =       item-name CFWS item-value

item-name       =       ALPHA *(["-"] (ALPHA / DIGIT))

item-value      =       1*angle-addr / addr-spec /
               atom / domain / msg-id
   A full discussion of the Internet mail use of trace fields is
   contained in [RFC2821].  For the purposes of this standard, the trace
   fields are strictly informational, and any formal interpretation of
   them is outside of the scope of this document.