I don't think you need a line in hosts.allow unless you are also using hosts.deny. The default is allow unless you specify otherwise.
I'm running qpopper on RH7 and have empty hosts.allow and hosts.deny files. More likely you have ipchains or iptables enabled and have not allowed port 110 through the firewall. >The documentation specifically states that you'll need to add a popper: >line to your /etc/hosts.allow. Read the PDF documentation for full >details, but in a nutshell you have a TCP wrapper starting the deamon >from xinetd. The wrapper will disallow connections from IPs it hasn't >been instructed to accept. For instance, my hosts.allow has a line >which reads like >popper: 172.16.20.2,10.0.0.3,10.0.0.4 >where the first IP is that of my mail proxy and the next two are IPs of >workstations on my LAN.
