Recently vie had a problem with email contacts sometimes receive email messages with a strange file attached, called winmail.dat. When they attempt to open this file, either it can’t be opened at all, or it contains "garbage" data.
When an Outlook user composes and sends a message using Rich Text Format or HTML Format, Outlook automatically generates a file, winmail.dat, and attaches it to the end of the message. winmail.dat contains formatting information, in a human-unreadable form, that Outlook will use on the receiving end to display this email message correctly. Unfortunately
there are a lots of blogs that told me to change my email to plain text or change my firewall settings none of these solved my problem.
In the end (totally frustrated I decided to look at our Exchange install for the solution, After digging for over an hour I found the MIME setting under Hub Transport role, I changed these to never use RTF and blow me, it started working, those users that could not receive the attachments suddenly were back OK.
Don’t know if this will help you as I’m not sure we had ever change this setting in the past but if it points you in the right direction….