nixos-config/modules/system/services/server/mailserver/roundcube/default.nix

{ config, ... }: 
{
  services.roundcube = {
    enable = config.system.mailserver.enable;
    hostName = "mail.nixfox.ca";
    extraConfig = ''
      $config['smtp_server'] = "tls://mx.nixfox.ca";
      $config['smtp_user'] = "%u";
      $config['smtp_pass'] = "%p";
    '';
  };
}