GSuite Email Delivery Issues - Possible DNS/WHM config Issue.

I manage a website and company email system that is having problems where outgoing emails are being marked as spam. Especially when sending to Outlook/Hotmail addresses. Emails to Google addresses seem to be ok and don't get marked as spam.

The company uses GSuite for their email system but there is also a CRM system that sends emails from a cPanel account on our VPS. Both systems are having this issue. The domain is not blacklisted and I suspect the issue is being caused by either incorrect DNS or server (WHM / CENTOS 7.8 kvm) configuration.

I have tried fixing it but I have hit a dead end. I need you to help me diagnose what is causing the issue and resolve it. I also need you to give our DNS a general health check to make sure its in good working order.

I can give additional information if requested but I would prefer to keep the domain private until you are awarded the job.

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Some Information from tests we have ran:

MX Toolbox:
- (Error) Primary Name Server Not Listed At Parent
- (Warning) DMARC Quarantine/Reject policy not enabled
- (Warning) Name Servers are on the Same Subnet
- (Warning) DMARC Quarantine/Reject policy not enabled
- (Warning) Reverse DNS Resolution FAILED! This is a problem
- (Warning) Reverse DNS does not match SMTP Banner

GSuite MX Check:
- All name servers should resolve to IP addresses.
- There was a network error getting TXT records for your domain. (says it can't find the SPF record which does actually exist)

Mail-Tester.com:
- 10/10 for an text email from a Gsuite account with no signature
- 7.9/10 for the same email from a Gsuite account but with a company logo/signature (logo hosted on Google Drive)
- 9.1/10 for a HTML email from our CRM (images hosted on our VPS)