Fix DirectAdmin Email Sending Issue

My DirectAdmin server suddenly stopped sending contact form submission mails after the emails were moved to a 3rd party provider (microsopft Exchange), and I need an experienced set of eyes to put things right.

I suspect the form submission email is not leaving the server environment.

What I need from you:
• Pin-point the exact misconfiguration in my DNS or email stack.
• Apply the correct fixes directly in DirectAdmin and, if required, the underlying Exim/Postfix configs.
• Verify SPF, DKIM, DMARC and reverse DNS all line up for clean delivery.
• Prove success by sending test emails to external providers (Gmail, Outlook, Yahoo) and sharing headers that show a passed status.
• Provide a brief, bullet-style hand-over note so I know precisely what changed and how to avoid the issue next time.

Root access to the server and DirectAdmin panel will be supplied after we agree to proceed. If you routinely work with DirectAdmin and DNS troubleshooting, this should be a quick, tidy fix.
There are two associated accounts this fix is to be applied to.