Because it didn't instruct me to modify /etc/hosts, I forgot to add
127.0.0.1 localhost
::1 localhost
to /etc/hosts. Thus, opensmtpd failed to start. It took a while to figure out the issue.
for @devs
editing etc/hosts really MISSING in our guide - see https://wiki.archlinux.org/index.php/Installation_guide#Network_configuration
we have only info about hostname.
Thanks for report, MilkCow
A default /etc/hosts actually exists in all ISOs (https://gitea.artixlinux.org/artix/iso-profiles/src/branch/master/common/root-overlay/etc/hosts) except base, but then again
there's no reason not to include it there too base is base.