Apple Mac OS X Server (Administrator’s Guide) - Manual (Page 401)

Mail Service 401
Rejecting Mail From Blacklisted Senders
You can have your mail service check an Open Relay Behavior-modification System (ORBS)
server to see if incoming mail came from a known junk-mail sender. ORBS servers are also
known as black-hole servers.
To reject mail from known junk-mail senders:
1 In Server Settings, click the Internet tab.
2 Click Mail Service and choose Configure Mail Service.
3 Click the Filter tab.
4 Select “Use a server for junk mail rejection” and then type the DNS name of an ORBS server.
5 Click Save.
Allowing SMTP Relay for a Backup Mail Server
If your network has more than one mail server, one can be designated as a backup server to
deliver mail in case the primary server goes down. (Backup mail servers are designated by MX
records.) A backup mail server may need to relay SMTP mail. You can set your server to ignore
SMTP relay restrictions when accepting mail as a backup server for another mail server.
To allow SMTP relay for a backup mail server:
1 In Server Settings, click the Internet tab.
2 Click Mail Service and choose Configure Mail Service.
3 Click the Protocols tab and choose Apple Mail Service SMTP from the pop-up menu.
4 Click SMTP Options.
5 Select “SMTP relay when host is a backup for destination” and click Save.
Filtering SMTP Connections
You can use the firewall service of Mac OS X Server to allow or deny access to your SMTP
mail service from specific IP addresses.
1 In Server Settings, click the Network tab.
2 Click Firewall and choose Show Firewall List.
3 Click New and configure the settings to create a filter that allows or denies access to port
number 25 from an IP address or range of IP addresses that you specify, then click Save.
Important
Blocking unsolicited mail from blacklisted senders may not be completely
accurate. Sometimes it can prevent valid mail from being received.