[Exim] failing address lookup

I have the following router and transport:

cyrususers:
no_verify
driver = manualrouter
caseful_local_part
local_parts = cdb;/usr/loca/exim4/cyrus-users.db
route_data = ${lookup{${local_part}}cdb{/usr/loca/exim4/cyrus-users.db}
transport = cyrus_smtp

cyrus_smtp:
driver = smtp
port = 25
gethostbyname

The router generates and address of "cyrus1" for the route_data. cyrus1 is in the
/etc/hosts file and in the dns and my nsswitch.conf has "hosts: files dns".

My problem is that every so messages get frozen like:

2003-10-30 11:06:31 1AFAdK-06TcTc-Mj <= ClusterPCs@??? H=(mercury.ex.ac.uk) [144.173.6.26] P=esmtp S=42557 id=E1AFAd7-00U1MS-Es@??? from <ClusterPCs@???> for ClusterPCs@???
2003-10-30 11:06:42 1AFAdK-06TcTc-Mj no IP address found for host cyrus1
2003-10-30 11:06:42 1AFAdK-06TcTc-Mj == NFBrooks@??? <ClusterPCs@???> R=cyrususers defer (-1): lookup of host "cyrus1" failed in cyrususers router
2003-10-30 11:06:42 1AFAdK-06TcTc-Mj Frozen

I don't understand two things. First I can't see why exim is failing to find the IP
address for cyrus1. Second, why is the message frozen and not just deferred and
re-tried later?

This is going to be a serious issue if I need to keep checking for this type of
frozen message. If I get eximon to deliver the message it goes through OK.

Any suggestions please?

Phil.
---------------------------------------
Phil Chambers (postmaster@???)
University of Exeter