Postfix stopped working when changing hostname

[mrtripps]

Hi I changed the hostname of my server in the plesk control panel. I now get errors in the /usr/local/psa/var/log/maillog when attempting to send email. The errors look like:

postfix/postfix-script: starting the Postfix mail system
postfix/master: daemon started -- version 2.8.4, configuration /etc/postfix
postfix/cleanup: fatal: host/service localhost/12768 not found: Name or service not known
postfix/pickup: warning: maildrop/E5559996219: error writing 4FA2C996217: queue file write error
postfix/master: warning: process /usr/libexec/postfix/cleanup pid 15334 exit status 1
postfix/master: warning: /usr/libexec/postfix/cleanup: bad command startup -- throttlin

Is there a setting in /etc/postfix/main.cf that I need to update?

Edit: the myHostName value matches the hostname I changed in the control panel. I have commented the following two lines in main.cf:

smtpd_milters = inet:localhost:12768
non_smtpd_milters = inet:localhost:12768

E-mail seems to be working ok again now. But can anyone tell me what those values are for? I don't want to leave them uncommented if they are meant to serve a purpose.