Validate SPF and DMARC by configuring DNS records.
In user Directadmin DNS records, obtain the generated SPF record for the mailserver (This varies on provider):
e.g

Copy and apply to cloudflare DNS management:

Wait ~10 minutes, if wanted, test with learndmarc, DMARC and SPF should pass.
Configured A record name mail to host IP address
and
configured MX record to @
e.g

based on https://developers.cloudflare.com/dns/manage-dns-records/how-to/email-records/
In email account list, click Enable DKIM

A domain key record in the DirectAdmin control panel will be generated. Replicate this into the Cloudflare DNS.

That should be everything! Woohoo!!!