In Our case the setting which seems to have fixed this was unticking Office 2007 export in the eDocprinter dialogue that Outlook was displaying when forwarding messages with PDF attachments.  It appears that when the eDoc printer driver was installed it made itself the default application for handling PDF and so Outlook was recreating the PDF files and converting them to winmail.dat at the same time, most odd.

One of our users is using Outlook to send emails via our mercury server.  attachments send by this user are winmail.dat attachments so other users need to use Fentun to save tham before they can open them which is a pain.  Is there a way to set Outlook not to do this?

http://support.microsoft.com/kb/278061

Thanks for that Brian.  I had a look at the users system and it is already set up to use HTML as per Microsoft's instructions for overcoming the problem.  Not the firs time I have found rubbish instructions on a Microsoft technical bulletin.

I have encountered many senders of winmail.dat attachments, most have fixed the problem after I sent them that link.  I assumed the others didn't fix it either because either they weren't tech savvy enough or their corporate IT folks wouldn't.  Perhaps there is more to it than that.  I will be interested to hear what you figure out.

Interesting reading here about how to handle the winmail.dat issue when the defaults don't work for specific addresses.

http://email.about.com/od/outlooktips/qt/Prevent_Winmail_dat_Attachments_from_Being_Sent_in_Outlook.htm