I wish to use openDKIM with Postfix mail server
on my Ubuntu xampp php site.

there are a lot of scaring details about turning to be one
of the "blacklist" domains if someone abuse my
mail server to send spam.

It won't be wise to create a domain name
in one of the public Internet DNS servers before
one masters security and antispam issues.

how can I setup openDKIM to use my IP instead of
using a domain name?

would the big mail companies accept my signature
(authentication) with IP and without domain name?

Is it a mandatory requirement to be registered in a DNS
in order to be accepted with openDKIM certificate?