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.