Outlook 2011 and the Rendering of Fonts pasted from other sources

I have a problem with one of my users.

When they construct an HTML email and copy text into it from another source it appears the same as in original source. Fine so far.

Then with outlook they reformat the copied text to the same as the rest of the email. E.g Arial. Outlook will change the text to the desired font.

They then send the email. Still fine so far.

When the email is opened on a windows machine however, the HTML formatting is ignored and it displays the formatting as if the user had never bothered to format the email to begin with. In short the copied text often looks like the user is "shouting" . Whereas the txt directly typed into the email appears correct.

My question is, why does this happen (the email appears correctly formatted on a mac) ? And how do i fix this. 

Thanks

Dafydd