There's a sendmail error log filling up with errors about this problem, although sendmail appears to be working OK.

I don't have a domain name for my site, just an IP address.

When I go to /etc/hosts I just see the name of the droplet twice and localhost:

127.0.1.1 nameofdroplet nameofdroplet
127.0.0.1 localhost

There's also a note saying I need to edit /etc/cloud/templates/hosts.tmpl

but there's no such file. It also suggests I could edit the value for manage_etc_hosts in /etc/cloud/cloud.cfg or "cloud-config from user-data". I don't know what that second one means and there's no entry for manage_etc_hosts in /etc/cloud/cloud.cfg.

Bit confused at this point. All I really need to do is reduce those constant errors filling up the log.