Configure Sendmail correct on Ubuntu Linux 8.10 dedicated server

I have an Ubuntu Linux 8.10 Dedicated Server and use Webmin as interface to the server. I want to use this server for bulkemailing. I have setup Sendmail as my smtp. The problem so far is that I can receive emails but when I want to send emails it fails. I am looking for someone to configure sendmail correctly so I can send emails to any email address that will be provided to me. I also want an explaination to what I did wrong for future references. After I have accepted a pro I will provide login details and all other asked for info.