The list is immense, and I didn't want to clutter my post with all the details. So just listing off things that spring to mind (because I don't know what OP doesn't know):

• Choosing an MTA - sendmail, postfix, exim, etc. and why you might choose one over the others
• Firewall settings
• Software/package management on your chosen distro
• Learning about DNS:
  • Host it - yourself via BIND
  • Or via a DNS service provider
  • DNS record types
    • Domains
    • Subdomains
    • A records/CNAMEs
    • MX records
    • Mail authority records - SPF's
    • Mail encryption records - DKIM
• Spam filtering, anti-virus
• Learning how to configure your MTA, which requires learning:
  • the configuration file language your MTA uses
  • what all the options mean and what they do
  • what the bare minimum options are to get up and running
  • how to make sure your configuration is secure and won't be exploitable by bad actors
  • how mail really gets delivered
  • how to setup secure smtp
  • how to set up SPFs
  • troubleshooting why GMAIL or Microsoft won't accept your mail
  • troubleshooting why GMAIL or Microsoft have stopped accepting your mail
  • dealing with blacklists/greylists when someone sends too many messages, or something that "looks too spammy"
• Mail hosting pitfalls
  • Being an open relay
  • Rate limiting
  • Reputation management
  • Vulnerabilities that let a hacker take over your server
  • Resource management - disk, memory, processes, queues, etc.
  • Downtime when you need to do updates
  • Downtime if you change your DNS configuration

I've definitely missed some stuff, and each of those things requires knowing other stuff too, so you can see that it's really a pretty deep subject. This is precisely why not many people self-host email themselves these days - the big guys have made it harder and harder to do so, in the name of eradicating spam, which they themselves are the biggest vectors for.