{
  config,
  pkgs,
  lib,
  ...
}: let
  mail_config = config.services.mailserver;

  vmail_config = mail_config.vmail_config;
  vmail_user = vmail_config.user;
  vmail_group = vmail_config.group;

  sieve_directory = mail_config.sieve_directory;

  virtualMailUsersActivationScript = pkgs.writeScript "activate-virtual-mail-users" ''
    #!${pkgs.stdenv.shell}

    set -euo pipefail

    # Create directory to store user sieve scripts if it doesn't exist
    if (! test -d "${sieve_directory}"); then
      mkdir "${sieve_directory}"
      chown "${vmail_user}:${vmail_group}" "${sieve_directory}"
      chmod 770 "${sieve_directory}"
    fi

    # Copy user's sieve script to the correct location (if it exists).  If it
    # is null, remove the file.
    ${lib.concatMapStringsSep "\n" ({
      name,
      sieveScript,
    }:
      if lib.isString sieveScript
      then ''
        if (! test -d "${sieve_directory}/${name}"); then
          mkdir -p "${sieve_directory}/${name}"
          chown "${vmail_user}:${vmail_group}" "${sieve_directory}/${name}"
          chmod 770 "${sieve_directory}/${name}"
        fi
        cat << 'EOF' > "${sieve_directory}/${name}/default.sieve"
        ${sieveScript}
        EOF
        chown "${vmail_user}:${vmail_group}" "${sieve_directory}/${name}/default.sieve"
      ''
      else ''
        if (test -f "${sieve_directory}/${name}/default.sieve"); then
          rm "${sieve_directory}/${name}/default.sieve"
        fi
        if (test -f "${sieve_directory}/${name}.svbin"); then
          rm "${sieve_directory}/${name}/default.svbin"
        fi
      '') (map (user: {inherit (user) name sieveScript;})
      (lib.attrValues mail_config.accounts))}
  '';
in {
  config = lib.mkIf (mail_config.enable) {
    users.users."${vmail_user}" = {
      isSystemUser = true;

      home = vmail_config.directory;
      createHome = true;

      uid = vmail_config.user_id;
      group = "${vmail_group}";
    };
    users.groups."${vmail_group}" = {gid = vmail_config.group_id;};
    systemd.services.activate-virtual-mail-users = {
      wantedBy = ["multi-user.target"];
      before = ["dovecot2.service"];
      serviceConfig = {ExecStart = virtualMailUsersActivationScript;};
      enable = true;
    };
  };
}