rfc2822 April 2001

4.5. Obsolete header fields

   Syntactically, the primary difference in the obsolete field syntax is
   that it allows multiple occurrences of any of the fields and they may
   occur in any order.  Also, any amount of white space is allowed
   before the ":" at the end of the field name.

obs-fields      =       *(obs-return /
              obs-received /
              obs-orig-date /
              obs-from /
              obs-sender /
              obs-reply-to /
              obs-to /
              obs-cc /
              obs-bcc /
              obs-message-id /
              obs-in-reply-to /
              obs-references /
              obs-subject /
              obs-comments /
              obs-keywords /
              obs-resent-date /
              obs-resent-from /
              obs-resent-send /
              obs-resent-rply /
              obs-resent-to /
              obs-resent-cc /
              obs-resent-bcc /
              obs-resent-mid /
              obs-optional)

   Except for destination address fields (described in section 4.5.3),
   the interpretation of multiple occurrences of fields is unspecified.
   Also, the interpretation of trace fields and resent fields which do
   not occur in blocks prepended to the message is unspecified as well.
   Unless otherwise noted in the following sections, interpretation of
   other fields is identical to the interpretation of their non-obsolete
   counterparts in section 3.