HTML part of the message is malformed by Outlook. It contains two different charset definitions.
I don't see those two different charset definitions.
I see:
------=_NextPart_001_004E_01CCD14A.5CD99450
Content-Type: text/plain;
charset="koi8-r"
Content-Transfer-Encoding: base64
This is ok!
I see:
------=_NextPart_001_004E_01CCD14A.5CD99450
Content-Type: text/html;
charset="koi8-r"
Content-Transfer-Encoding: quoted-printable
This one is also ok.
Then I see that there is HTML part which one has:
charset=3Dwindows-1251">
And looks like here can be the problem as that part above is really in Windows-1251 format.
Where interesting problem.
Will try to look into it.