How To Fix Postfix Warning: dict_nis_init: NIS domain name not set - NIS lookups disabled

First of I didn’t ask postfix to do NIS lookups. These warning messages started showing up when I commented out alias_maps and decided to only use alias_database instead. The fix was simple.

alias_maps when commented, uses the default settings:
alias_maps = hash:/etc/aliases, nis:mail.aliases

This allows postfix to work even without explicit alias_maps settings. However the default also encumbers it with checking nis. Setting alias_maps explicitly eliminates the problem:
alias_maps = hash:/etc/aliases