Disable TNEF, Fix WINMAIL.DAT Issue with MS 365

Enter the following command in powershell after connecting to 365. This will fix issues with various external mail clients receiving winmail.dat attachments for all mail leaving 365 (default)

Set-RemoteDomain Default -TNEFEnabled $false

An alternative is to disable TNEF to certain recipient domains with the following commands:

New-RemoteDomain -Name flash.net -DomainName flash.net

The above adds a new recipient domain.

Set-RemoteDomain -Identity flash.net -TNEFEnabled $false

Disable TNEF for selected domain.

Get-RemoteDomain -Identity flash.net | Select TNEFEnabled

Check TNEF status.