What is Outlook framing?

Question

Character encoding

We can’t find it, we’ll honor Outlook knocks down the framing when answering. There are checkboxes "Reply in the same encoding" and by default they tried UTF-8, cp-1251, etc. etc.
Here is a piece of the letter with the heading:
This part of the letter is displayed correctly!

------=_NextPart_000_002B_01D0A911.C4FDA600
Content-Type: text/plain;
  charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

=D0=92=D0=B0=D0=BB=D0=B5=D1=80=D0=B8=D0=B9, =
=D0=B7=D0=B4=D1=80=D0=B0=D0=B2=D1=81=D1=82=D0=B2=D1=83=D0=B9=D1=82=D0=B5.=

=D0=A1=D1=80=D0=B5=D0=B4=D0=B0, 17 =D0=B8=D1=8E=D0=BD=D1=8F 2015, 11:11 =
+03:00 =D0=BE=D1=82 [email protected]###.ru:

just below is the broken encoding that appeared during the response (the text that the recipient received):

$B'5'S'Q'X'Q'V'^'Q'q(J $B'/'Q'd'Q']'n'q(J $B'#']'Q'U'Z'^'Z'b'`'S'_'Q(J, =
$B'Y'U'b'Q'S'c'd'S'e'['d'V(J.

It feels like the hexadecimal code was formed in ASCII, and the "=" characters were converted to '.