How to stop Office 365 sending winmail.dat attachments to a non Microsoft mail client (Mac mail or Thunderbird for example).

This is often due to Office 365 or Exchange sending emails in a proprietary format named TNEF (Transport Neutral Encapsulation Format) that only other Microsoft clients accept.

This can be disabled for your tenant via powershell with the following commands:

1) $LiveCred = Get-Credential
2) $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri -Credential $LiveCred -Authentication Basic -AllowRedirection
3) Import-PSSession $Session
4) New-RemoteDomain -Name *name* -DomainName *domain*
5) Set-RemoteDomain *name* -TNEFEnabled $false
6) Remove-PSSession $Session

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