Mails from Apple are shown malformed in Outlook 2010 (Windows) I have a setup with Mac and Windows clients. The Mac users use Apple Mail and all users have an HTML signature with images. When a Mac user replies to an email the text from the initial message becomes an attachment with the name: untitled attachment001.html and the image from the signature becomes an attachment with the name: PastedGraphic.pdf.

Can this behaviour be avoided? How can I preserve the original email and signature?

I have managed the following. I discovered that it was due to the malformed signature. So I created a .webarchive file that contains the signature as I want to be. The signature appears correctly at both the file and the apple mail signature editor. But when I open to write a new mail only the borders of the signature image appear and there is an '?' inside the borders. What does this mean?
The Mac users need to make sure that their outgoing email format is HTML or Rich Text even if the incoming format was plain text or use a plain text signature.

I'd prefer the latter: I use plain text email whenever possible as it's more-widely compatible and usually is easier to read. This, by necessity, would entail using plain text signatures.