[Exim] :fail: text not being returned on SMTP connections.

Hi I'm running Exim 4.14 I have aliases set with :fail: however the
message after the :fail: is not being returned during the SMTP phase.
They're getting the generic "unknown user" from the ACL acl_check_rcpt.
My aliases router is:

system_aliases:
driver = redirect
allow_fail
allow_defer
data = ${lookup{$local_part}dbm{/etc/exim/db/aliases.db}}
file_transport = address_file
pipe_transport = address_pipe

How do I get the fail message returned during SMTP time in Exim4?

Rule testing returns the appropriate text, and the SMTP transactions log
to MAIN with a 550 and the appropriate text.

Please let me know if I have omitted a significant peice of information.

Thank you very much.

-rob