I have used simple-nixos-mailserver (not reviewed here) for three years now, for myself and several organizations. NO issues. The biggest issue with e-mail is (

I have used simple-nixos-mailserver (not reviewed here) for three years now, for myself and several organizations. NO issues. The biggest issue with e-mail is (in my view at least) your IP reputation. In order to ameliorate this, I simply buy colocation space, which has higher quality IPs than AWS and cloud services. I've never had deliverability issues. Because it's nix, upgrades are atomic. Installation is dead simple. Also, the entire setup was like two minutes. I eventually added LDAP authentication and roundcube which took some work, but because it's nix, I did this development on a separate VM before I simply replaced my config on my main server (actually just copied the closures). Adding domains is simple: just modify the nix config file. As is adding new users. Very easy. Highly recommend to run all your critical infrastructure on Nix. Any mistakes are easy to fix... just go back to the old generation. I did consider solutions like mail-in-a-box but adding the complexity and statefulness of such a system just didn't seem worth it. Also, remote control panels scare me. My e-mail server is heavily SSH protected (Only my private key plus wireguard) and hardened against malicious IPs (fail2ban) among other things. Again, NixOS makes this easy. For storage, simple-nixos-mailserver keeps all data on a separate ZFS partition, which gets zfs 'send't to a remote backup (several).