I helped a client find a solution to the “winmail.dat” email attachment issue.  I had seen this a few times before, it’s rare, but resolvable.
The problem is that you are using Outlook (I assume) to send attachments.  You are sending them as Rich Text format.  Normally HTML is used and won’t cause problems.  Some  webmail programs do not like RTF.  The other major one is AOL.  So, A. It can be corrected on your end if you are in fact using Outlook to send mail.
http://www.pchell.com/support/winmaildat.shtml
How to Configure Outlook NOT to send Winmail.dat attachments
To Turn off Rich Text sending for messages in Microsoft Outlook
1) Click on Tools
2) Click Options, and then click the Mail Format tab.
3) In the Send in this Message Format list, select Plain Text, and then click OK.
This will set your default sending method to Plain Text, which will lose your special formatting options with fonts, colors, etc. However everyone, no matter what email program they are using, will now be able to receive your email with no problems.  

B.   I installed a “.dat” convertor type program.  It’s a few extra steps, but it works.
http://www.kopf.com.br/winmail/download.php

Kurt Rein
Mobile Computer Wizard
858 345-0382 Cel
619 255-1215 Office