Did you just upgrade your existing 10.1.5 installation to 10.2?
If so, I suspect that your mail server is now incorrectly configured and sendmail took over mail services instead of postfix.
Things to check:
*Make sure postfix is running: sudo lsof -i | grep master
*If it isn't running start it manually: sudo postfix start
*If it isn't running, this means that your postfix startup item was overwritten in the upgrade process. If this is the case, you will have to recreate the startup item.
*Make sure /System/Library/StartupItems/Sendmail/Sendmail has an "exit 0" at the beginning of the script.
*Make sure /etc/hostconfig has the line: MAILSERVER=-YES-