You can’t control the view a recipient used when sending a message. Your recipients could be using any email system, some of which don’t even support HTML messages. Others support HTML, but have it turned off by default.
Depending on the nature of the HTML you imported, it is also possible GroupWise may make some automatic modifications to it as part of the send process (but not enough to convert it to plain text). We would be happy to take a look at your stationery if you email it firstname.lastname@example.org.