taken as-is from eggplant
for portability in other hosts, not listing 'eggplant.jenkins-ci.org'

according to http://manpages.ubuntu.com/manpages/hardy/man8/update-exim4.conf.8.html,
dc_other_hostnames specify the local domains. Since we aren't using eggplant.jenkins-ci.org as a valid host name portion of any e-mail address, this change should be safe.
exim4 tries to read this file after it does setuid

So the file needs to be readable to 'Debian-exim4' user
defined another flavor we use
deploying DKIM to eggplant