An email being recieved perfectly by a client from their own ESP, finds that when they then forward this email the new recpipient gets an email with the whole of the original email converted to a single image! Like a screen shot (although not accuate as things link links have changed colour, etc)

I have never heard of this, nor can I reproduce it.

He has forwarded to me, and it is a PNG called image002.png on desktop and my iPhone... although, he says he only gets the image version when viewed on mobile (so maybe forwarding to his own Outlook doesn't cause it to change).

What is causing this and can it be avoided?