Always think twice when you give smtp access to your clients. Stay away from spammers and always add necessary rules and policy for your service, in order to cancel your client's account when it is really become vital.

how are you able to monitor your clients account? I am just wondering.

Make sure you don't have open relays, and always limit the mail recipient size to under 50 so that nobody spams using your smtp server.

Otherwise you can be listed in spam databases and get blacklisted everywhere that uses global spam databases in their smtp settings.

Also, check your IIS bad mail folder everyday and see if one of your clients has used your CDONTS component to send spam.

I usually check www.dnsstuff.com a couple of times a week to make sure none of my mail servers' IP address are on any blacklists. Takes about 5 minutes but can definitely help in the long run

Yes, dnsstuff.com also allows you to see whether your mail server has reverse dns record or not. Some mail servers require that you have a reverse dns record in order to send them emails.