I received the following error message - does anyone know what it means?

You haven't really given enough info here.

Best guess though - it's a CSF LFD alert complaining about one of your mail processes running longer than a default value.

You could edit the /etc/csf/csf.pignore to ignore whichever postfix processes it's alerting on. It probably had the executable listed along with the 3 pieces of info you provided above.
If it was smtpd, the line would look something like

exe:/usr/libexec/postfix/smtpd