How do we do this - DKIM, SPF, DMARC Setup?

Problem: We have a Virtualmin server that supports multiple domains and we are having trouble sending some of the emails. Running an mxtoolbox.com scan, we fail on three tests:

dmarc - DNS Record Not Found mx - No DMARC Record found smtp - Reverse DNS is not a valid host name

Here's the test for one of our accounts: https://mxtoolbox.com/domain/dallaswomenlawyers.org/

Setup: Virtualmin is up-to-date and is running on CentOS. BIND is turned off and DNS is handled by the data center. The server name does not match the all of the domains that are hosted.

Question:
1. Where do we start to correct these failures?

2. Does the DMARC need to be setup in the data center's DNS?

3. What should the SMTP reverse DNS look like for each domain?

Thanks for any help.