Had a customer who is using Exchange 365 as their mail host. All of a sudden people who they were emailing started to receive the winmail.dat attachments instead of the correct email. Now we all know that winmail.dat in incompatible message formats (See KB: 278061) in between mail hosts, but how do we fix it without having to update the clients or address books to send mail to particular clients in HTML or Plain Text?

After investigating this the solution seemed to be fairly easy as you can see below, it seems this issue was being caused by Exchange sending the message in TNEF format. Here are the steps.

Connect to Exchange 365 via PowerShell

$LiveCred = Get-Credential

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

Once connected Import the commands

Import-PSSession $Session

Once the commands have downloaded we want to turn of TNEF on the default domain

Set-RemoteDomain Default -TNEFEnabled $false

This can then be checked by running

Get-RemoteDomain |fl