The Exim plugin maintains three kinds of files in domain configuration directories:
aliases, e-mail aliases to be concatenated into /etc/aliases.hosted
mail files that, if non-empty, should contain the current domain's name, indicating that the current node should accept mail for that domain for local delivery
mail.relay files that, if non-empty, should contain the current domain's name, indicating that the current node should provide relaying for any mail addressed to that domain
A post-handler performs the concatenation over all domains into /var/domtool/aliases, /var/domtool/local_domains.cfg, and /var/domtool/relay_domains.cfg, respectively; and then runs domtool-publish exim, which copies the first file to /etc/aliases.hosted and concatenates the rest to /etc/exim4/conf.d/main/10_domtool-domains, and then runs /etc/init.d/exim4 reload.