<p>I have a customer who needs to send some direct mailing to his contacts which is being collected from trade shows etc. I am thinking about providing a web based solution for them on sending direct mailing material- however this kind of solution always face a major difficulty on having the server blacklisted. I would like to know if there is any suggestions on how to maintain such SMTP server?</p>

Aside from the server (just set up exim or postfix and a web interface to send newsletters from, and do this on a server with an IP address that is different from your own personal mail server (maybe even a different network)), you will want to get your terms and conditions of use nice and tight so that you can remove access to the systems from any customer that has complaints made about emailing. Indeed you can mandate that all emails sent must be lawful (e.g., unsubscription option that works, etc).