A co-worker of mine was having problems receiving only winmail.dat attachments on email sent though Outlook. I had the sender cc my email address and was getting the same winmail.dat file. We made some changes to the sender's outgoing Outlook account and I started receiving regular .xls Excel attachments. My co-worker is still getting winmail.dat. I sent an email to him and cc myself and he was able to open the attachment, but then I got the winmail.dat. The previous sender resent to both him and myself and my co-worker got the winmail.dat and I got the attachment just fine. Does anyone have any idea why the same email will open fine on one machine and not the other, and sporadically at that? We are both using newer Macbook Pros (and fully up to date) and OSX Mail. The emails are being sent from Windows machine running Outlook. Using Outlook OSX on my Macbook gives the same results as OSX Mail. I'm not looking for work arounds to the problem. We both have the TNEF app to open winmail.dat files and our emails have to be sent in HTML format to keep the corporate email signatures intact and properly formatted. Any insight is appreciated and as always, thanks in advance. Edit: If I open the same emails on my Windows machine in Outlook the attachments all work.