Have you recently migrated to O365 and are getting reports from your users about winmail.dat attachments? Well you’re not the only one. This is somewhat common and typically caused by a mail client that doesn’t support TNEF. I think it’s also worth noting that this is not a scenario specific only to Office 365.
Here’s Microsoft’s explanation:
Do some of your users report that e-mail recipients in external domains can’t open their messages that contain a Winmail.dat attachment? If so, the recipients in the external domain are probably using an e-mail client that doesn’t support the Transport Neutral Encapsulation Format (TNEF). Microsoft Outlook is one of the few e-mail clients that support TNEF-encoded messages, although some third-party utilities can help convert Winmail.dat attachments.
So how do you stop this? I’ll make this simple for you, copy and paste the following in an administrative PowerShell. Spacing should be correct below, thus no need for running each line individually.
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri # -Credential $LiveCred -Authentication Basic -AllowRedirection
Set-RemoteDomain Default -TNEFEnabled $false
Don’t forget to disconnect your session
Brandon Stuart, PEI