Simply turning of mail services in Server Admin does not prevent Postfix from starting up again when needed/called by another process.
To completely disable Postfix and Cyrus on Mac OS X Server 10.5.x/10.4.x issue:
sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/org.postfix.master.plist
sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/edu.cmu.andrew.cyrus.master.plist
Should you have the need to re-enable these services just issue:
sudo /bin/launchctl load -w /System/Library/LaunchDaemons/org.postfix.master.plist
sudo /bin/launchctl load -w /System/Library/LaunchDaemons/edu.cmu.andrew.cyrus.master.plist