でも6.7のWARNING TO IMPLEMENTORSの段落に

> If binary data is encoded in quoted-
> printable, care must be taken to encode CR and LF characters as "=0D"
> and "=0A", respectively. In particular, a CRLF sequence in binary
> data should be encoded as "=0D=0A". Otherwise, if CRLF were
> represented as a hard line break, it might be incorrectly decoded on
> platforms with different line break conventions.

とあるので、"should"の解釈にもよるけど、Mewだけじゃなく、
バイナリ中のCRLFをhard line breakにしちゃうOutlook側も改善の余地ありか。