Is there a remote domain defined for that MDaemon server?
You could set the remote domain content type to MimeHtml and see if that helps.
otherwise, I tend to blame the receiving server or client for these issues, not a transport rule.
If you were to BCC another remote contact in that rule, I would wonder what format it is for that receiving client.
https://learn.microsoft.com/en-us/powershell/module/exchange/set-remotedomain?view=exchange-ps