Problem when Notes forward an Exchange multipart message

Hello,

I really have a strange problem to solve.

On our company, we are using Exchange2003/Outlook to send HTML messages with PDF files attached to some colleagues in another division.

This division use Domino server.

They receive our messages without any problems, and they forward them, every day to some external contacts That use Exchange.

From time to time, and for no obvious reasons, the external contacts receives Messages sources with base64 code instead of files attached.

Any ideas ?

Thanks

FG