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No! The MIME RFCs specifically state that lines (in header or body)
which use MIME encoding are limited to 76 (N.B. not 78 or 998) characters.
Please do not suggest, imply, or leave to the reader's imagination that
MIME-encoded text can be placed in long lines. Do not under any
circumstances imply that MIME-conforming implementations must handle
encoded text in lines "at least up to the 998 character limit".
Section 2.3 states in part (I'll address the other part in a separate message):
o Lines of characters in the body MUST be limited to 998 characters,
and SHOULD be limited to 78 characters, excluding the CRLF.
Note: As was stated earlier, there are other documents,
specifically the MIME documents ([RFC2045], [RFC2046], [RFC2049],
[RFC4288], [RFC4289]), that extend this specification to allow for
different sorts of message bodies. Again, these mechanisms are
beyond the scope of this document.
Note (as stated above) that the MIME RFCs *limit* line lengths to 76
characters when MIME encoding is used. The text "extend this specification"
might be misinterpreted (76 characters is a *limitation*, not an
*extension*) w.r.t. line length limits. At a minimum, some clarification
of this point is needed in the draft text.