{
  config,
  lib,
  pkgs,
  ...
}: let
  inherit
    (lib)
    mkIf
    mkEnableOption
    mkOption
    ;
  inherit (lib.types) listOf str;
  cfg = config.services.mathebau-mail;
in {
  options.services.mathebau-mail = {
    enable = mkEnableOption "mathebau mail service";
    domains = mkOption {
      type = listOf (lib.types.submodule {
        options = {
          domain = mkOption {
            type = str;
          };
          allowlistPass = mkOption {
            type = str;
          };
          virt_aliases = mkOption {
            type = str;
            default = "";
          };
        };
      });
    };
  };

  config = mkIf cfg.enable {
    environment.systemPackages = [pkgs.alias-to-sieve];

    services = {
      stalwart-mail = {
        enable = true;
        openFirewall = true;
        settings = {
          server = {
            lookup.default.hostname = "fb04184.mathematik.tu-darmstadt.de"; # Because the DNS PTR of 130.83.2.184 is this and this should be used in SMTP EHLO.
            listener = {
              "smtp" = {
                bind = ["[::]:25"];
                protocol = "smtp";
              };
              "submissions" = {
                # Enabling sending from these domains privately blocked on https://github.com/stalwartlabs/mail-server/issues/618
                bind = ["[::]:465"];
                protocol = "smtp";
                tls.implicit = true;
              };
              "imaptls" = {
                bind = ["[::]:993"];
                protocol = "imap";
                tls.implicit = true;
              };
              "management" = {
                bind = ["[::]:80"]; # This must also bind publically for ACME to work.
                protocol = "http";
              };
            };
          };
          acme.letsencrypt = {
            directory = "https://acme-v02.api.letsencrypt.org/directory"; # This setting is necessary for this block to be activated
            challenge = "http-01";
            contact = ["root@mathebau.de"];
            domains = ["fb04184.mathematik.tu-darmstadt.de" "imap.mathebau.de" "smtp.mathebau.de"];
            default = true;
          };
          spam.header.is-spam = "Dummyheader"; # disable moving to spam which would conflict with forwarding
          auth = {
            # TODO check if HRZ conforms to these standards and we can validate them strictly
            dkim.verify = "relaxed";
            arc.verify = "relaxed";
            dmarc.verify = "relaxed";
            iprev.verify = "relaxed";
            spf.verify.ehlo = "relaxed";
            spf.verify.mail-from = "relaxed";
          };

          # Forward outgoing mail to HRZ or mail VMs.
          # see https://stalw.art/docs/smtp/outbound/routing/ relay host example
          queue.outbound = {
            next-hop = [
              {
                "if" = "rcpt_domain = 'lists.mathebau.de'";
                "then" = "'mailman'";
              }
              {
                "if" = "is_local_domain('', rcpt_domain)";
                "then" = "'local'";
              }
              {"else" = "'hrz'";}
            ];
            tls = {
              mta-sts = "disable";
              dane = "disable";
              starttls = "optional"; # e.g. Lobon does not offer starttls
            };
          };
          remote."hrz" = {
            address = "mailout.hrz.tu-darmstadt.de";
            port = 25;
            protocol = "smtp";
            tls.implicit = false; # somehow this is needed here
          };
          remote."mailman" = {
            address = "lobon.mathebau.de"; # must be created in DNS as a MX record
            port = 25;
            protocol = "smtp";
            tls.implicit = false; # somehow this is needed here
          };

          # In order to accept mail that we only forward
          # without having to generate an account.
          # Invalid addresses are filtered by DFN beforehand.
          session.rcpt = {
            catch-all = true;
            relay = [
              {
                "if" = "!is_empty(authenticated_as) || rcpt_domain == 'lists.mathebau.de'";
                "then" = true;
              }
              {"else" = false;}
            ];
          };
          config.local-keys =
            [
              "store.*"
              "directory.*"
              "tracer.*"
              "server.*"
              "!server.blocked-ip.*"
              "authentication.fallback-admin.*"
              "cluster.node-id"
              "storage.data"
              "storage.blob"
              "storage.lookup"
              "storage.fts"
              "storage.directory"
              "lookup.default.hostname"
              "certificate.*"
            ] # the default ones
            ++ ["sieve.trusted.scripts.*"]; #for macros to be able to include our redirection script
          sieve.trusted.scripts.redirects.contents = "%{file:/tmp/virt_aliases}%";
          session.data.script = "'redirects'";

          authentication.fallback-admin = {
            user = "admin";
            secret = "$argon2i$v=19$m=4096,t=3,p=1$d0hYOTkzclpzSmFTZUplWnhVeWE$I7q9uB19RWL0oZKaPlMPSlGfFp6FQ/vrx80FFKCsalg";
          };
        };
      };
    };
    environment.persistence.${config.impermanence.name} = {
      directories = [
        "/var/lib/stalwart-mail"
      ];
      files = ["/root/.ssh/known_hosts"]; # for the backup server bragi
    };

    # Update HRZ allowlist
    # For account details see https://www-cgi.hrz.tu-darmstadt.de/mail/
    # will stop working if no valid TUIDs are associated to our domain.
    systemd = {
      timers."mailAllowlist" = {
        wantedBy = ["timers.target"];
        timerConfig = {
          OnBootSec = "5m"; # Run every 5 minutes
          OnUnitActiveSec = "5m";
          RandomizedDelaySec = "2m"; # prevent overload on regular intervals
          Unit = "mailAllowlist.service";
        };
      };
      services = {
        "mailAllowlist" = {
          description = "Allowlist update: Post the mail addresses to the HRZ allowllist";
          script = let
            scriptTemplate = {
              domain,
              allowlistPass,
              ...
            }: ''
              # Get the mail addresses' local-part
              ${pkgs.stalwart-mail}/bin/stalwart-cli --url http://localhost:80 -c $(cat /run/secrets/stalwartAdmin) account list | grep '@${domain}' | sed 's/| //' | sed 's/ |//' | tee /tmp/addresses
              ${pkgs.stalwart-mail}/bin/stalwart-cli --url http://localhost:80 -c $(cat /run/secrets/stalwartAdmin) list list | grep '@${domain}' | sed 's/| //' | sed 's/ |//' | tee -a /tmp/addresses
              ${pkgs.stalwart-mail}/bin/stalwart-cli --url http://localhost:80 -c $(cat /run/secrets/stalwartAdmin) group list | grep '@${domain}' | sed 's/| //' | sed 's/ |//' | tee -a /tmp/addresses
              ${pkgs.gnugrep}/bin/grep -o -e "[A-Za-z0-9.!#\$%&'*+-/=?^_{|}~]*@${domain}" /tmp/virt_aliases | tee -a /tmp/addresses # This doesn't catch all RFC conform local parts. Improve if you need.
              # Post local-parts to HRZ
              ${pkgs.curl}/bin/curl https://www-cgi.hrz.tu-darmstadt.de/mail/whitelist-update.php -F emaildomain=${domain} -F password=$(cat ${allowlistPass}) -F emailliste=@/tmp/addresses -F meldungen=voll
              # Cleanup
              rm /tmp/addresses
            '';
          in
            lib.strings.concatStringsSep "" (map scriptTemplate cfg.domains);
          serviceConfig = {
            Type = "oneshot";
            User = "stalwart-mail";
            NoNewPrivileges = true;
            # See https://www.man7.org/linux/man-pages/man5/systemd.exec.5.html
            PrivateTmp = false; # allow access to sieve script
            ProtectHome = true;
            ReadOnlyPaths = "/";
            ReadWritePaths = "/tmp";
            InaccessiblePaths = "-/lost+found";
            PrivateDevices = true;
            PrivateUsers = true;
            ProtectHostname = true;
            ProtectClock = true;
            ProtectKernelTunables = true;
            ProtectKernelModules = true;
            ProtectKernelLogs = true;
            ProtectControlGroups = true;
            LockPersonality = true;
            MemoryDenyWriteExecute = true;
            RestrictRealtime = true;
            RestrictSUIDSGID = true;
          };
        };
        "stalwart-mail" = {
          restartTriggers = lib.attrsets.mapAttrsToList (_: aliaslist: aliaslist.sopsFile) config.sops.secrets;
          serviceConfig.PrivateTmp = lib.mkForce false; # enable access to generated Sieve script
        };
        "virt-aliases-generator" = {
          description = "Virtual Aliases Generator: Generate a sieve script from the virtual alias file";
          script = let
            scriptTemplate = {
              domain,
              virt_aliases,
              ...
            }:
              if virt_aliases != ""
              then "${virt_aliases} ${domain} "
              else "";
          in
            lib.strings.concatStringsSep "" (["${pkgs.alias-to-sieve}/bin/alias_to_sieve "] ++ map scriptTemplate cfg.domains ++ ["> /tmp/virt_aliases"]);
          wantedBy = ["stalwart-mail.service"];
          serviceConfig = {
            Type = "oneshot";
            User = "stalwart-mail";
            NoNewPrivileges = true;
            # See https://www.man7.org/linux/man-pages/man5/systemd.exec.5.html
            PrivateTmp = false;
            ProtectHome = true;
            ReadOnlyPaths = "/";
            ReadWritePaths = "/tmp";
            InaccessiblePaths = "-/lost+found";
            PrivateDevices = true;
            PrivateUsers = true;
            ProtectHostname = true;
            ProtectClock = true;
            ProtectKernelTunables = true;
            ProtectKernelModules = true;
            ProtectKernelLogs = true;
            ProtectControlGroups = true;
            LockPersonality = true;
            MemoryDenyWriteExecute = true;
            RestrictRealtime = true;
            RestrictSUIDSGID = true;
          };
        };
      };
    };
    # Backups
    services.borgbackup.jobs.mail = {
      paths = [
        "/var/lib/stalwart-mail/data"
      ];
      encryption.mode = "none"; # Otherwise the key is next to the backup or we have human interaction.
      environment = {
        BORG_RSH = "ssh -i /run/secrets/backupKey";
        # “Borg ensures that backups are not created on random drives that ‘just happen’ to contain a Borg repository.”
        # https://borgbackup.readthedocs.io/en/stable/deployment/automated-local.html
        # We don't want this in order to not need to persist borg cache and simplify new deployments.
        BORG_UNKNOWN_UNENCRYPTED_REPO_ACCESS_IS_OK = "yes";
      };
      repo = "borg@192.168.1.11:kaluut"; # TODO for https://gitea.mathebau.de/Fachschaft/nixConfig/issues/33
      startAt = "daily";
      user = "root";
      group = "root";
    };
  };
}