I had to allow external SMTP access. I am using SSL to secure things a bit. What I would like to do just as a bit of insurance from someone using my system to spam, I would like to limit the number of messages that can be sent per a given amount of time.
For example, I had an ISP that limited you to 50 outgoing emails per hour. This would be great on a per user basis, but I could live with it being the whole server.