Unreadable e-mail and attachements

A colleague of mine often receives unreadable e-mail with HTML code and attached Word or Excel files that show up in the mail text with a bunch of unreadable symbols. When she looks at the same mail in a browser via WebMail, they look completely correct.
When the original sender re-sends these e-mails, most of them work properly the second time. It does not make a difference if the file comes from a PC or a Mac. The problems are not reproduceable. It seems to be totally by chance.
She uses an Intel MacBook with OS 10.4 (in French) with Apple Mail and an IMPA account.

*****************The messages with HTML code look, for instance, like this:

This is a multi-part message in MIME format.
------= NextPart_000_000401C7ADB4.CE44C380
Content-Type: multipart/alternative;
boundary="----= NextPart_001_000501C7ADB4.CE473480"
------= NextPart_001_000501C7ADB4.CE473480
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
=20
=20
Subject:=20
=20
Hallo Nathalie,

Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<html-tag<
<head-tag<
META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; =>
charset=3Diso-8859-1">
meta name=3DGenerator content=3D"Microsoft Word 10 (filtered)">
<style-tag<
!--<br> /* Font Definitions */
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */


*****************Then later body and div tags also follow...

*****************The attached file looks for instance, like this:

------= NextPart_000_000401C7ADB4.CE44C380
Content-Type: application/vnd.ms-excel;
name="statistique EB 06 07.xls"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="statistique EB 06 07.xls"
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***************** Any ideas or help?
Thank you