I found a rather obscure bug with Leopard while troubleshooting my postfix logs. It seems that mail that I had been getting from some of the mail lists I usually receive was being bounced by my server because it couldn't resolve the IP address based on the DNS supplied by the sending server. If you've read my prior Postfix postings, you know that I'm fairly strict about the servers I accept mail from, and misconfigured servers generally don't get any mail delivered here.
So, this came as somewhat of a surprise that formerly working servers were now being rejected after my upgrade to Leopard. Some troubleshoot assistance from the postfix mail list uncovered the issue, the getnameinfo function in the OS was not resolving DNS addresses that resolved to a CNAME record, or anything other than a PTR record. The unix nslookup and host commands though, worked fine, but postfix relies on the getnameinfo function.
The good news here is that this bug has been reported to Apple, and signs are good that this should be fixed in Mac OS X 10.5.2 when it is released. I'll report back on that after release. For now, my temporary workaround is to identify servers that aren't resolving, and whitelist them in my helo_access list.