I have another post for this problem, but I think I lost the guy who was working with me. The link is below. Long story short: I installed the latest GroupWise service pack and ever since none of my users can print plain text emails.
Here is the error we get: this file does not have a program associated with it for performing this action. Create an association in the Set Associations control panel. In some cases you can hit the reply button and then print. I have noticed that this works for users with their default compose set to HTML.