[suse-security] How can I solve this problem with mail server

Hi to all, My company mail server is running on a Suse 6.3 system, with sendmail 8.9.3, and until sotime ago everything was running beautifully. What happened is that we started to receive mail messages with links to porno sites. These mail messages are always addressed to non existant users in the mail system. We have tried to configure sendmail by way of webadmin to reject mail from the source addresses but the source adress is always changing. Is there any security problem with Suse 6.3 and/or sendmail that can cause this ? What can I do to solve this? Thanks in advance // luis oliveira