When a user runs mailtool(1) on a remote machine, setting the DISPLAY environment back to the local machine, this message might appear inside a dialog box window. The message also indicates that the Classing Engine must be installed to use Attachments. This problem occurs because rlogin(1) does not propagate the user's environment.
Exit mailtool(1) and set your OPENWINHOME environment variable to /usr/openwin. Then run mailtool(1) again. The error message does not appear, and you can now use Attachments.
Classing Engine is a new name for Tool Talk. Earlier versions of mailtool(1) said Tool Talk: TT_ERR_NOMP instead of Classing Engine.