[Exim] Unrouteable domain or not?

I've just upgraded from exim3 to exim4.05 and currently I'm watching
the logs carefully to see if everything's working (which by and large
it is). There are some things going on however I don't understand.

Take the following log entries. The first recipient (msg 17URfp)
(which is a forward from our system), fails saying the domain
processrequest.com is unrouteable. Yet the following 2 messages
(17URfP and 17URfQ) work fine. What's going on here? Why is
processrequest.com unrouteable for the first user but not for the
other two (paranoia: have I fouled up somewhere in converting from
exim3->4).

Glenn

17URfP-0000Pc-00 <= listman@???
H=dal1bs070.processrequest.com (DAL1BS070) [216.39.67.120] P=smtp
S=5067
17URfP-0000Pc-00 ** d.e.sh@??? <du@???>
R=dnslookup T=remote_smtp: SMTP error
from remote mailer after MAIL FROM:<listman@???>
SIZE=6209: host dirc.bris.ac.uk [137.222.10.51]: 550 Unrouteable do
main 'ProcessRequest.com'
17URfP-0000Pc-00 Completed
17URfQ-0000Pc-00 <= listman@???
H=dal1bs070.processrequest.com (DAL1BS070) [216.39.67.120] P=smtp
S=5073
17URfQ-0000Pc-00 => rg@??? <robert@???>
R=dnslookup T=remote_smtp H=green.csi.cam.ac.uk [131.111.8.57]
17URfQ-0000Pc-00 Completed
17URfR-0000Pc-00 <= listman@???
H=dal1bs070.processrequest.com (DAL1BS070) [216.39.67.120] P=smtp
S=5075
17URfR-0000Pc-00 => slim@??? R=localuser T=local_delivery
17URfR-0000Pc-00 Completed