The SunView mailtool(1) and prior 3.3 OpenWindows mailtool(1) produce this message when they cannot cope with an attachment. The attachment is probably in MIME (multipurpose internet mail extensions) format, using base64 encoding.
To read a mail message containing MIME attachments, use mailtool(1) from a system running at least the Solaris 2.3 release. If you are running an earlier version of the Solaris environment, rlogin(1) to a system running a later version, set the DISPLAY environment variable back to the first system, and run mailtool remotely. If those options prove impossible, ask the originator to send the message again using mailtool(1), or using the CDE dtmail compose File->SendAs->SunMailTool option.
Standard MIME attachments with base64 encoding, for example, produce this message and fail to display in older mailtool(1)s.
Look into using metamail, available on the Internet, which allows you to send and receive MIME attachments.