I'll check the firewall it may be blocking port 113 entirely, although in that case I would expect failure with an appropriate error message (Ident Failed?).

Another question, how does courier handle lookups for private addresses? Many people now use private addresses locally, so dnslookup for a fully qualified domain name would fail but the local name service would succeed. Does it use the operating systems name lookup, (as defined in nsswitch) which in my case would succeed through the hosts table in the server which also serves a number of other functions on our network (wins).