Dreaded Winmail.DAT

Hi Guys,

I have a problem with users (externally) on Apple (crappy Mail) Client receiving Winmail.dat attachments from my domain. I fully understand that this is a TNEF issue with e-mail clients that are not on Microsoft Exchange (backend) or on Outlook (front ends). So please don't treat this question or prescribe the usual Microsoft KB article as a solution because I've tried that and it has not worked thus far.  

Just fyi:
Here is are my backend specs: Exchange 2010 with all of my users sending emails are on either Outlook 2010 or Outlook 2011 for Macs.

Now, one of my external clients having the issue with my domain sent attachments has given me access by creating me an account on their system which is a generic hosting company using POP and IMAP to relay the emails along to Apple Mail client. I've requested this account to test sending attachment with from my domain to theirs.

During my testing I've tried the numerous recommendations by Microsoft and others here in this forum to no avail. But here is the question that I cannot answer.

For some odd reason that I am not able to explain yet the new account created for me does not exhibit the problem and attachments reach my account on that system correctly but another person (who created the account for me) has issue with my email address and receiving the Winmail.DAT attachments when I sent him the emails.

This person advised me that at first when he started receiving emails from my domain there were no issues with attachments and that it begun happening after a while but the problem has not changed since it begun happening.

If the issue was really my Outlook client then it would be as simple as sending HTML or Plain Text emails and the problem would be solved. Furthermore, my new account on that system has not had any problems with the test attachments that I am sending but my colleagues account is,  both his apple mail and iphone are showing Winmail.DAT when sending emails.

Should I look into my Exchange server settings? What should my next steps be?

Please note: The I've also turned off any SPAM filtering used at the clients end to ensure that it is not an issue.

Please help,

Confused IT Manager :-)