I have a setting on my DNS record for default_domainkey that automatically populated when I added my site with kualo.com hosting. Do I need to use default_domainkey? If so, what settings should I use for it?
That sounds like a DKIM record which is used for authenticating email. If you use email on your domain, it is sensible to use DKIM which is why that record is there.
Your mail host should provide the DKIM record needed.
Thank you @domjh !