Outlook 2000: user can't receive attachments from few acco..

Hi, I'm working for 3 days on a very strange problem and since now I
haven't found a solution.

The situation is this:
<SENDER> sends a mail (RTF or HTML) with and attachment (.DOC) to 2
users of our company.
<USER1> receive the mail right formatted with attachment
<USER2> receive the mail in plain text with no attachment. Same using
OWA

We use Exchange and I traced the mail since the frontend to the
mailbox, but I can't find anything wrong.

These are parts of the mail headers:

<USER1>
------_=_NextPart_001_01C76003.ACE33B3A
Content-Type: multipart/alternative;
boundary="----_=_NextPart_002_01C76003.ACE33B3A"

------_=_NextPart_002_01C76003.ACE33B3A
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

------_=_NextPart_002_01C76003.ACE33B3A
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable


------_=_NextPart_002_01C76003.ACE33B3A--
------_=_NextPart_001_01C76003.ACE33B3A
Content-Type: application/msword;
name="file.doc"
Content-Transfer-Encoding: base64
Content-Description: file.doc
Content-Disposition: attachment;
filename="file.doc"


------_=_NextPart_001_01C76003.ACE33B3A--




<USER2>
------_=_NextPart_001_01C76003.ACE33B3A
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

------_=_NextPart_001_01C76003.ACE33B3A
Content-Type: application/ms-tnef;
name="winmail.dat"
Content-Transfer-Encoding: base64


------_=_NextPart_001_01C76003.ACE33B3A--



As you can see the USER2 header don't have same content-type section
of USER1.
It's present insted a generic tnef winmail.dat section.

Somehow it seems that che mime content isn't "parsed" in the right
way, but I can't understand how and why.
Suggestions ?