Iām running Mailcow for this purpose. Mailcow is a complete mailserver including postfix, dovecot, rspamd, webmail, and it gives you the lines to enter in dns, like spf, dmarc, etc.
Mailcow runs as docker compose containers and easy to set up. I know that Mailcow has an easy way to install and integrate Nextcloud in it. Just by starting an script.
This might be a good alternative to manual install for some of you.
I run Mailcow for some time now, and it is very stable. Before I used the manual install too, with lots of configurations and (security) management.
Thank you, @PancakeConnaisseur. I have two servers set up using your tutorials at your website (123qwe.com), and they work great. I like the use of Postgresql because of the same reasons you mentioned concerning mariadb/mysql having some quirks to it. ***Edit: this is unrelated, but there is a great tutorial here: Host your own mailserver with Dovecot, Postfix, Rspamd and PostgreSQL | Pieter Hollander that configures a mailserver with DNSSEC. He also has some other very cool tutorials. You might to be able to add a Nextcloud configuration on top of his tutorial. I should mention this site also: Mailserver mit Dovecot, Postfix, MySQL und Rspamd unter Debian 10 Buster [v1.0]