I ran into troubles with exim4 on debian taking forever to start. Strace'ing it revealed it was trying to look itself up using DNS, ignoring what was in /etc/hosts and /etc/nsswitch.conf. I was testing a box before I hooked to a network that had a DNS server running. Commenting out everything in /etc/resolv.conf took care of things. What if I only wanted exim4 for local delivery, which I did? BTW, it also makes logging in with ssh a lot faster as sshd is doing reverse lookups.