Error message AUP#EML-038

What on earth does this error message mean, returned on trying to forward an email that I have received?

This error happens perhaps once every so many months.  It is not a common occurrence.  There is something obviously wrong with the email that I am trying to forward.  

It does not matter how I try to forward the message, whether I use forward, redirection or send as an attachment.  What I can do is print the message to a pdf, and send that.  That works.

It has nothing to do with the email addresses I am sending to.

So, what is the problem?  I'd like to let the sender of the email know.