Remove winmail.dat file from Office 365
This is an issue with Hosted Exchange. The fix required logging into the hosted exchange using powershell and running the following:
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
Set-RemoteDomain Default -TNEFEnabled $false
Remove-PSSession
Remove-PSSession $Session