Having your own mail server could be useful but also sometimes dangereous. I am happy to see appliance such as iRedMail which cover variaty of topics regarding a somehow complete solution. I pick Ubuntu 22 on Hetzner. First you create DNS A record for your mail server and following by this a MX record pointing at that A record. Be sure to set proper hostname in the system. You can check it with:

hostname -f

Ensure you have it set also in /etc/hosts and /etc/hostname. Next download iRedMail installer and iRedMail.sh script. It will prompt for various things but in my case I was missing dialout package, so be sure to install it before running installation. I choose PostgreSQL and NGINX backends as I am more aware of them than MariaDB and Apache. Once the installer finishes it is required to reboot your system to apply all the settings.

Administration panel is available at mail.yourdomain.ext/iredadmin. Webmail at mail.yourdomain.ext/mail. Now if all went fine you will be able to create additional user accounts and send messages to your mailboxes. However you will not be able to send it to external mail servers such as Gmail because you lack of security and antispam configuration on your DNS side. So…

To setup SPIF create TXT DNS entry saying which IP addresses are eligable to send messages from. If you do not care that much, then create record allowing any server to send:

yourdomain.ext. 3600 IN TXT "v=spf1 mx -all"

This is however not enough for several mail providers and you need to setup also DKIM record. It utilizes digital signature using public key. Not to going too much deep into the topic setting it up is also quire easy:

amavisd showkeys

Grab command output and create another DNS TXT entry:

v=DKIM1; p=verylongkeystringwithoutquotes

So with these two DNS mail server verification features you will be able to send messages to external servers without them complaining about your setup. iRedMail documentation explains both basic installation and this DNS confguration quite nice so be sure to check it out also.