I had not seen this issue for a long time. However, I received an email from a client recently telling me that his recipients using Apple Macs could not open his attachments, as they were received as WINMAIL.DAT files.
This is due to Office365 setting the default attachment format to TNEF. This format causes issues with mail clients that are unable to read this attachment format, thus creating the WINMAIL.DAT issue.
The solution is to use PowerShell to make changes to the Office365 account. Refer to my previous post here on how to set up PowerShell for Office365.
- $Cred = Get-Credential
- Connect-msolservice -credential $Cred
- Type in your Office365 admin user credentials
- $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $Cred -Authentication Basic -AllowRedirection
- Import-PSSession $Session
- Set-RemoteDomain Default -TNEFEnabled $false
- Remove-PSSession $Session
That should do the trick. The Microsoft help article documenting this can be found here – http://help.outlook.com/140/gg263346.aspx
[Edit 11 May 2015: Changed commands for connecting to Office365]