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.