We have a development server for a client that was recently upgraded from Tiger Server to Leopard Server. This system holds the subversion repository and the staging sites for their hosted application. One of the configured pieces is that whenever someone commits into the SVN repository, we have a post-commit hook that sends a message to all the developers with the information from the revision commit. Email on this system is handling by the Apple built-in Postfix. When the system was upgraded, we noticed that we no longer received our SVN commit messages. Investigating this I found two things that needed fixing.
My first problem was that the logging that postfix was sending to syslogd was very sparse. so I checked through all the settings twice in Server Admin and directly in the main.cf and master.cf files. Took me a while, but I finally looked at the /etc/syslogd.conf file and found that the facility entry for mail was set to mail.warn. Checked the Server Admin setting for SMTP log level and set it to Debug.
Second problem: now that logging was fixed, I can see that the relayhost set in config was rejecting the messages. So not only were the original messages being rejected, the bounce messages were being bounced. Essentially anything being send was dying a quick death. I fixed the relayhost setting, tried another messages and BAM, message delivered.
Upgrading from Tiger to Leopard is an important step to take, but with all upgrades, one must really go through all your settings once again to verify their correctness.