[CentOS] preventing apache from being a mail relay

I am trying to recall back at least 2 years, and my notes are poor, and my searching appears to be worst...

Seems I recall that last when I set up my apache server, the spammers were posting to it so it would send out the spam on port 25. There was some conf that I did to block this, but I did not document it, and I can't find any reference to this.

I don't think my memory is that bad, but it IS sunday...

I don't want to put up this new server and have it flooding the world with spam and then get the server blocked. So do I remember correctly that this was a problem? Is it still, and how is this prevented?

Thanks. Am putting up better notes this time around.