Big question: why would you want to run your own mail server?
Maybe you want to run a professional looking website for a budding business, and everybody else does it? Maybe it's just to be able to share the accomplishment? Maybe you just want to have a way to generate endless accounts under your own domain for filtering email into buckets or connections.

Regardless of the reason, you need to understand that this is a commitment not just to yourself and any potential users, but also to the entire internet. No exaggeration internet-accessible email hosts are the origin for much of the world's SPAM. The setup and administration of an email server is not for the lazy or feint of heart, it is not a trivial endeavour. So be prepared.

With that warning in place, let me add that a mail server supporting only internal (to the server) traffic is relatively simple and a decent way to monitor local server operations. A lightly enlarged system that can accept traffic between local computers and isolated from the internet is also a relatively safe and simple task. But when you open up the gateway to the internet all kinds of things can happen.

So figure out how far you want this to go. That will make the selection of software and firewall settings easier to identify as you go through the configuration.

I am veering away from writing my own "how to do email" philosophy for two reasons.
First, the installation and setup is more difficult and less automated than any other server task
Second, the links just below already cover all the nitty-gritty details and are probably better written than any of mine.
If you're building a CentOS/RedHat 8 or 9 email server, the following pages will get you where you want to go.

1. Setup the MTA Run Your Own Email Server on CentOS 8/RHEL 8 – Postfix SMTP Server
2. Setup the MDA Set up Dovecot IMAP server and TLS encryption
3. Adding Flexibility Create Virtual Mailboxes with PostfixAdmin
4. For internet use Creating SPF and DKIM record to get through spam filters
5. For internet use Setting up DMARC to protect your domain reputation
6. For internet use How to Stop Your Emails From Being Marked as Spam
7. Additional Flexibility How to Host Multiple Mail Domains in PostfixAdmin
8. For internet useBlocking Email Spam with Postfix
9. For internet useBlocking Email Spam with SpamAssassin
10. For internet useSetting Up Amavis and ClamAV to Check Viruses in Email Messages
11. For internet useHow to Secure Email Server Against Hacking with Self-hosted VPN Server
12. For internet useHow to Bypass Email Blacklists
13. For internet useEnable and Configure Postscreen in Postfix to Block Spambots
14. For internet useAutomatic IP and Domain Warm-up For Your Email Server
15. For internet useHow to Set Up Postfix SMTP Relay on CentOS/Rocky Linux with Sendinblue