Most machines define localhost as 127.0.0.1 and so they are equivalent but some machines define localhost as some other loopback address (like 127.0.1.1), and on those machines they are not equivalent. Since we use localhost and 127.0.0.1 in different parts of the setup, we may be configuring things inconsistently.

We should use "localhost" consistently throughout.

See https://discourse.mailinabox.email/t/opendkim-is-not-running-port-8891/1188/12.