If your log starts filling with entries about "problem talking to server private/anvil", chances are you did a straight upgrade from your previous Mac OS X Server version to 10.5
When updating from Mac OS X Server 10.3 or 10.4 to 10.5, your existing /etc/postfix/master.cf will be preserved. This per se is not bad in itself.
10.5 however, comes with a new version of Postfix which has new capabilities and expects a few more parameters. Should you happen to see above message repeatedly in your logs, check if /etc/postfix/master.cf contains an entry starting with "anvil".
If it doesn't, edit /etc/postfix/master.cf and below:
lmtp unix - - n - - lmtp
anvil unix - - n - 1 anvil
When done, stop and start mailservices.