OS-X Leopard comes pre-installed with a Postfix version. No need to install it via darwin ports or other such mess (actually uninstall it if you have previously manually installed it via ports or something similar). Postfix just needs to be enabled and following sequence of several easy steps explains how to do it:

 sudo vi /System/Library/LaunchDaemons/org.postfix.master.plist

add following line before the closing </dict> tag:

<key>RunAtLoad</key> <true/> <key>KeepAlive</key> <true/>

Start service with

$ sudo launchctl
launchd% start org.postfix.master

You can also start simply with “sudo postfix start” but the above command will start via launch sequence and test the configuration you edited in the previous step.

Check that SMPT is running:

telnet localhost 25

For more information, see: Mac OS X Manual Page For launchd.plist