MIME: multipart/mixed

Hi
Has anybody ever used the MIME "multipart/mixed" content type in a script? Does it work? I´m currently having trouble with it and can't figure out what´s going on.

I have two different email contents, one for some plain text and one for a pdf attachment. Each one works perfectly used independantly with respective MIME content types, but when trying to mix these in one single email using "multipart/mixed" I only get an empty mail with an invalid pdf attachment.

Any suggestion would be very appreciated (last time seemed like I scared everybody with MIME, got no reply )
/Yann