Trying to avoid being an open SMTP relay

Leopard 10.5.4 Server on a Dual 2GHz G5. Have enabled SMTP/Allow Incoming Mail, and am trying to avoid inadvertently being an open relay for junk mail. Steps taken so far:
1. Am running Sieve with avelsieve plugin enabled for SquirrelMail server-side mail filtering;
2. Am requiring SMTP authentication (Kerberos);
3. In “Relay” section of Server Admin Mail Settings, have specified “Accept SMTP relays” only for our own local network;
4. Also in “Relay” section of Server Admin Mail Settings, have specified various suspicious IP addresses in the “Refuse all messages from…” section;
5. Have also entered those suspicious IP addresses in the Server Firewall to refuse any connections from them.
I think all of the above should prevent us being an open relay, but in the “Maintenance/Mail Queue” section of Server Admin, I still see maybe 40 or 50 messages per day stacked up that seem to be from some of the very IP addresses entered in “Refuse” dialogs, above. When I select one of those messages and then click, “delete,” frequently they ALL delete simultaneously.
Is this normal or am I not doing all I could to prevent this apparent junk mail from getting in?