What do people use to monitor that their email server is accepting external email? Most monitoring solutions send notices by email which is useless when incoming email breaks, so some other method of notification is required. Ideally this is a piece of software that we can just run locally, but external services are also acceptable.
Background: Our ADSL modem has a stupid firewall that blocks incoming connections, which can be removed manually via telnet but comes back whenever the PPPoE connection drops. We're going to buy a new one soon, but having monitoring for email seems like a good idea generally.