A customer has two offices - one in the UK, one in Australia. They share a common TLD for their email. All inbound email passes through the cloud based FuseMail filtering service, and FuseMail forward emails to either the UK or Australian Exchange servers based on group membership.
When users in Australia send an email to a specific user in the UK, the UK users gets any embedded images or attachments as a winmail.dat attachment. If other UK users are CC'd into the same email, then those recipients see the images in the email and have the correct attachments, but this one user still has just a winmail.dat attachment. And it is ONLY when receiving emails from the Australian users.
On the UK Exchange server, the customer is also using Exclaimer for adding signatures and disclaimers to emails.
I cannot figure out WHAT is causing this one recipient to keep getting the attachments as winmail.dat.
Just to add to this, it doesn't matter if this one user uses Outlook (in cached mode), OWA, his tablet or iPhone - in all cases, when he gets emails from the Australian users, he always ends up with winmail.dat attachments (even when other recipients do not).