Sysadmin Is it too late to run Setup Step 1...two years later?

After having a working host for 2 years, I noticed that I never set the default server main hostname properly and I'm now running into email DMARC issues for server sent root emails. When I run:

Code:

postconf -d myhostname mydomain

I get:

Code:

myhostname = mycorrecthostname.localdomain
mydomain = localdomain

Similarly, running:

Code:

uname -a

doesn't show a proper FQDN full main hostname.

I now know this is my fault because I simply never understood nor ran Step 1 (Setup The Default Server Main Hostname Nginx Vhost).

My dilemma is, with multiple working domains on this server, whether it's too late to go back and go through Step 1 in the Getting Started Guide?

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I should have also mentioned that I'll be setting up all my DNS records (reverse PTR, SPF, DKIM, DMARC) after I solve this default server main hostname issue.