Exchange Online – avoid winmail.dat attachments and configuring TNEF settings


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Having headaches with the winmail.dat file being sent instead of PDF’s, for example, with Exchange Online.

The issue truly comes to light during a hybrid staged migration. During the transition, Outlook clients that are connected to Exchange Online will use a setting to send using HTML for outside recipients. Well, since you are in hybrid mode, and your company has a fully populated GAL courtesy of Directory Sync, this conversion never occurs.

Solution

There is a simple way to fix this problem is to set the default remote domain send policy to disable TNEF. Connect to Powershell:

$LiveCred = Get-Credential

In the Windows PowerShell Credential Request window, type the credentials of an account in your cloud-based organization. Then, click OK.

Run the following command:

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection

Once your connection to Exchange online Powershell is completed run the following command:

enable-organizationcustomization

Once this is completed run the following command:

set-remotedomain Default -TNEFEnabled $false

This will fix the winmail.dat send issue while in hybrid. I don’t see a specific need to change this after the transition. Most Outlook clients will set to use HTML for internet recipients, therefore, this will not likely effect you again.

TNEF settings on Exchange Server

TNEF settings can be accessed and managed through the Exchange Management Console > Organization Configuration > Hub Transport > Remote Domains or using the following cmdlets:

Exchange Online – avoid winmail.dat attachments and configuring TNEF settings

To get a list of all remote domains where TNEF is disabled:

Get-RemoteDomain | Where {$_.TNEFEnabled -eq $false}

To create a new remote domain:

New-RemoteDomain -DomainName thedomain.com -Name thedomain

To enable TNEF:

Set-RemoteDomain -Identity thedomain -TNEFEnabled $true

To disable TNEF:

Set-RemoteDomain -Identity thedomain -TNEFEnabled $false