For people using 365 and send a lot of email to Mac people. Mac users can received email with attachment winmail.dat but they can’t open it.
This behavior occur when user within Outlook send email in a RTF format.
To force a conversion from RTF to HTML, connect to Office 365 with PowerShell (check on my blog for the login information).
Then run the following
Set-RemoteDomain Default -TNEFEnabled $false
The command will convert the RTF to HTML to all domains.