Detection of incoming SMTP connections..iptables question ?

Hello, I am seeking for a solution to the following. I use an iptables based firewall which works more or less... I am trying to find a way of detecting that *no* incoming SMTP connection has occurred for a certain period .. say 5 minutes. I need a program to take a certain action if this occurs, so it has to be magical. I can interrogate the logs of course, but I wonder if any wizard out there has a neater solution. Thanks -- Regards Cliff