[exim] false unknown user errors

My relay server is bouncing messages with "unknown user" errors from a
single virtual domain on a server that has multiple virtual domains. I
am confident the users do exisit on the server. All other domains are
working fine. I am sure I will not get any assistance on the far end
and need to get passed the error without disabling lookahead. What are
my options?

Ubuntu 12.04, Exim4, Baruwa 2.0


Raymond