rfc2822 April 2001

3.6. Field definitions

   The header fields of a message are defined here.  All header fields
   have the same general syntactic structure: A field name, followed by
   a colon, followed by the field body.  The specific syntax for each
   header field is defined in the subsequent sections.

   Note: In the ABNF syntax for each field in subsequent sections, each
   field name is followed by the required colon.  However, for brevity
   sometimes the colon is not referred to in the textual description of
   the syntax.  It is, nonetheless, required.

   It is important to note that the header fields are not guaranteed to
   be in a particular order.  They may appear in any order, and they
   have been known to be reordered occasionally when transported over
   the Internet.  However, for the purposes of this standard, header
   fields SHOULD NOT be reordered when a message is transported or
   transformed.  More importantly, the trace header fields and resent
   header fields MUST NOT be reordered, and SHOULD be kept in blocks
   prepended to the message.  See sections 3.6.6 and 3.6.7 for more
   information.

   The only required header fields are the origination date field and
   the originator address field(s).  All other header fields are
   syntactically optional.  More information is contained in the table
   following this definition.

fields          =       *(trace
                *(resent-date /
                 resent-from /
                 resent-sender /
                 resent-to /
                 resent-cc /
                 resent-bcc /
                 resent-msg-id))
              *(orig-date /
              from /
              sender /
              reply-to /
              to /
              cc /
              bcc /
              message-id /
              in-reply-to /
              references /
              subject /
              comments /
              keywords /
              optional-field)

   The following table indicates limits on the number of times each
   field may occur in a message header as well as any special
   limitations on the use of those fields.  An asterisk next to a value
   in the minimum or maximum column indicates that a special restriction
   appears in the Notes column.

Field           Min number      Max number      Notes

trace           0               unlimited       Block prepended - see
                                      3.6.7

resent-date     0*              unlimited*      One per block, required
                                      if other resent fields
                                      present - see 3.6.6

resent-from     0               unlimited*      One per block - see
                                      3.6.6

resent-sender   0*              unlimited*      One per block, MUST
                                      occur with multi-address
                                      resent-from - see 3.6.6

resent-to       0               unlimited*      One per block - see
                                      3.6.6

resent-cc       0               unlimited*      One per block - see
                                      3.6.6

resent-bcc      0               unlimited*      One per block - see
                                      3.6.6

resent-msg-id   0               unlimited*      One per block - see
                                      3.6.6

orig-date       1               1

from            1               1               See sender and 3.6.2
sender          0*              1               MUST occur with multi-
                                      address from - see 3.6.2

reply-to        0               1

to              0               1

cc              0               1

bcc             0               1

message-id      0*              1               SHOULD be present - see
                                      3.6.4

in-reply-to     0*              1               SHOULD occur in some
                                      replies - see 3.6.4

references      0*              1               SHOULD occur in some
                                      replies - see 3.6.4

subject         0               1

comments        0               unlimited

keywords        0               unlimited

optional-field  0               unlimited

   The exact interpretation of each field is described in subsequent
   sections.