I just discovered that, ever since I updated this computer to kernel-2.6.32-431.3.1, it hasn't been forwarding outbound email.
I found this repeated in /var/log/maillog: "warning: relayhost configuration problem"
This computer is set up to relay outbound email via postfix to another computer on my network by name, whose name and IP address is specified in the /etc/hosts file.
A bit of reading told me that I have to add this line to /etc/postfix/main.cf:
smtp_host_lookup = dns,native
Without that line, postfix won't check /etc/hosts to look up an IP address.
Oddly, this setup has been working for several months WITHOUT that line in main.cf and it just stopped working when the latest kernel was installed. According to the documentation http://www.postfix.org/postconf.5.html#smtp_host_lookup it appears that it should not have been working before, eithre. Something changed in the new kernel to make it stop working.