Specify 'Text-Only' Mails for certain receipients

Some time ago we’ve changed the mime outgoing conversion setting to ‘plain text and html’, because the mails were not sent to the internet as formatted. Noew there works everything fine.

The problem is that we use a parser service in the internet that MUST receive the messages ‘text only’.

Is there a way to force a mailrouter to send mails to a certain receipient/domain as text only?

Thanks in advance