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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 $false
This will fix the winmail.dat send issue while in hybrid. I don’t see a specific need to change this after the transition. Most Outlook clients will set to use HTML for internet recipients, therefore, this will not likely effect you again.
TNEF settings on Exchange Server
TNEF settings can be accessed and managed through the Exchange Management Console > Organization Configuration > Hub Transport > Remote Domains or using the following cmdlets:
To get a list of all remote domains where TNEF is disabled:
Get-RemoteDomain | Where {$_.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
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