For easy setup of mailserver we can go declarative way with nixos - we will go through setting up fully working mailserver instance together with proper DNS records.

nixos mailserver is nix package containing these services:

• Postfix & Dovecot
• rspamd (spam filter) & clamav (virus scanning)
• opendkim
• sieve custom scripts
• user aliases & catch all & regular aliases

For proper deliverability we first of all need to configure reverse dns record (PTR) (this domain is also used as fqdn in this how-to). With this we can proceed to configuration on vps (placed in configuration.nix config file) accordingly:

(below shown configuration is my own slightly modified for showcase purposes of functionality. So please dont forget to change values (highlighted) according to your needs.

{ config, pkgs, ... }:
{
  imports = [
    (builtins.fetchTarball {
      url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/v2.3.0/nixos-mailserver-v2.3.0.tar.gz"; # for up-to-date tar please follow gitlab repository on link above
      sha256 = "0lpz08qviccvpfws2nm83n7m2r8add2wvfg9bljx9yxx8107r919";
    })
  ];


# sets up path to sieve scripts
#  services.dovecot2.sieveScripts = {
#    before = "/etc/nixos/mailserver/sieve/vpsf.sieve";
#  };

  mailserver = {
    enable = true;
    fqdn = "<your-rDNS-ready-domain-here>";
    domains = [ "martinmyska.cz" "domain.cz" "domain2.cz"]; # here we specify hosted domains

    # A list of all login accounts. To create the password hashes, use
    # mkpasswd -m sha-512 "super secret password"
    loginAccounts = {
        "martin@martinmyska.cz" = {
            hashedPassword = "<your-sha512-password-here>";

            aliases = [
                "myska@martinmyska.cz" # alias where to look for other emails (not a mailbox address)
            ];

            # Or we can set this to catch all mails going to whole martinmyska.cz domain.com
            catchAll = [
                "martinmyska.cz"
            ];
        };
        "info@domain.cz" = {
            hashedPassword = "<your-sha512-password-here>";

            # Or we can catch all mails going to completely another domain
            catchAll = [
                "domain2.cz"
            ];
        };


    };
    # Extra virtual aliases. These are email addresses that are forwarded to
    # loginAccounts addresses.
    extraVirtualAliases = {
        # address = forward address;
        #"abuse@example.com" = "user1@example.com";
    };

    # Use Let's Encrypt certificates. Note that this needs to set up a stripped
    # down nginx and opens port 80.
    certificateScheme = 3;

    # Enable IMAP and POP3
    enableImap = true;
    enablePop3 = true;
    enableImapSsl = true;
    enablePop3Ssl = true;

    # Enable the ManageSieve protocol
    enableManageSieve = true;

    # whether to scan inbound emails for viruses (note that this requires at least
    # 1 Gb RAM for the server. Without virus scanning 256 MB RAM should be plenty)
    virusScanning = true;
  };
}

At last we need to properly configure our domain DNS records as follows: