Yes, I'm pretty sure its TNEF.
Start with the first question and work down - we may have the answer before we get to the last...
Is RTF your default? (Shouldn't be, use HTML instead.)
Does MYOB use RTF messages? Does MYOB use custom template? (See bug below)
The copy in your sent folder: is it RTF or HTML?
Do the recipients receive an HTML or plain text message? How big is the message they get - a few KB or large enough to have the attachment?
One of two things could be the cause:
RTF message format results in a winmail.dat which is not readable except by Outlook.
MYOB uses a custom template and you are sending to an interent address. There is a bug where messages created using a custom template and sent using SMTP are converted to plain text. If this was the problem, they should receive the attachments (I think) - when i tested it with an HTML template that had pictures, the pictures were attached to the plain text message. Maybe its different if there is a real attachment, rather than embedded images. (This is on for fixing in June I think.)
Regardless, its not clickyes's doing.

They just push buttons. If you use an antivirus scanner on your email it could be the cause too - removing the winmail.dat so even outlook users get just a plain text body, but that is less common. (Or it removes the attachment, leaving HTML body).