A home system with an email server, i.e. exim, need not lay "exposed" 24/7. Is there a way to write script to open a port such as SMTP/25 periodically for a certain amount of time, check for activity, wait till free and then close it. This would be a cron'ed equivalent of bringing up Guarddog or some other IPtables interface, enabling access, waiting a while and seeing no (or no more) activity, bringing it up again and disabling access.