How to configure Exchange Server (2013/2016) to convert RTF messages in HTML format

Exchange 2013/2016 or Exchange Onlin

-Open the Exchange Management Shell.

-To see you current settings, run the command:

            Get-RemoteDomain | fl

                      Get-RemoteDomain | Where {$_.TNEFEnabled -ne $false}

- For each Remote domain, run the command:

                       Set-RemoteDomain -Identity * -TNEFEnabled $false
                      Set-RemoteDomain -Identity * -ContentType MimeHtml

After configuring the Exchange server in this way, messages sent by users in RTF will be converted to HTML by the server.

https://technet.microsoft.com/en-us/library/bb310786(v=exchg.150).aspx
https://technet.microsoft.com/en-us/library/aa998600(v=exchg.150).aspx