Done some extensive testing. Currently you'll have this in the second box of the first triplet in the Exim Configurator:
Code:
message = "The recipient cannot be verified. Please check all recipients of this message to verify they are valid."
verify = recipient
If you get rid of the message line, so that you just have:
then the message in /etc/valiases will be used in the failure error to the sender.
However, if there is no message after the :fail: in /etc/valiases, there will be a blank 550 message in the failure error.
I did play with coding it to cope with this issue through a perl script, which worked, but didn't really complete it.