RFC 2046, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", November 1996

Note: This RFC has been updated by RFC 2646, RFC 3798, RFC 5147, RFC 6657, RFC 8098

Errata ID: 6776
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Brian Antos
Date Reported: 2021-12-05

Section 5.1.1 says:

NOTE:  The CRLF preceding the boundary delimiter line is conceptually
   attached to the boundary so that it is possible to have a part that
   does not end with a CRLF (line  break).  Body parts that must be
   considered to end with line breaks, therefore, must have two CRLFs
   preceding the boundary delimiter line, the first of which is part of
   the preceding body part, and the second of which is part of the
   encapsulation boundary.

It should say:

NOTE:  The CRLF preceding the boundary delimiter line is conceptually
   attached to the boundary so that it is possible to have a part that
   does not end with a CRLF (line  break).  Body parts that must be
   considered to end with line breaks, therefore, must have two CRLFs
   preceding the boundary delimiter line, the first of which is part of
   the preceding body part, and the second of which is part of the
   encapsulation boundary.

The requirement that the encapsulation boundary begins  with
a  CRLF  implies  that  the  body of a multipart entity must
itself begin with a CRLF before the first encapsulation line
--  that  is, if the "preamble" area is not used, the entity
headers must be followed by TWO CRLFs.  This is  indeed  how
such  entities  should be composed.  A tolerant mail reading
program, however, may interpret a  body  of  type  multipart
that  begins  with  an encapsulation line NOT initiated by a
CRLF  as  also  being  an  encapsulation  boundary,  but   a
compliant  mail  sending  program  must  not  generate  such
entities.

Notes:

Recommend re-introducing the wording from the original RFC (1341) regarding preceding CRLF and the first delimiter line. Current RFC is ambiguous about handling the initial line without it.

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