Multipart Emails to Gmail and Yahoo Don't Work

[devel95]

I am sending multipart/mixed. When testing against Gmail and Yahoo, I can see both the plain text and the html versions in the browser window. The attachment part works just fine, but it the combination of the appearance of the 2 other multiparts that I need to resolve.

Does anyone have an idea why my multipart email does not work with Gmail or Yahoo? Thanks.

Here is an extract of the corresponding headers:

MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="MultipartBoundary_2290f28aef87a0e27094cf342e192cc9"

This is a multi-part message in MIME format.

--MultipartBoundary_2290f28aef87a0e27094cf342e192cc9
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

[html message here]

--MultipartBoundary_2290f28aef87a0e27094cf342e192cc9
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

[plain text message here]

--MultipartBoundary_2290f28aef87a0e27094cf342e192cc9
Content-Type: application/vnd.ms-excel; name="test.xls"
Content-Disposition: attachment; filename="test.xls"
Content-Transfer-Encoding: base64

[file attachment here]

--MultipartBoundary_2290f28aef87a0e27094cf342e192cc9--