qmail security...

Hey guys,

I use qmailtoaster on one my mail server and it works great, i have a few questions and here are they.

1. How do i secure my qmail server from relaying, spammers and what are the most available security threats to a mail servers.

qmail toaster comesup with SpamAssasin.. but still i get a lot of unwanted mails in my inbox.. , is their any way that i can block 'em all.

smtp_auth is enabled and i checked the relay tests and my server never uses relays.. but still wanna make sure no one gets access.. what security measure do i need to take care.