http://www.postfix.org/BDAT_README.html
says to stop logging of disabled ehlo keywords add "silent_discard" like
/etc/postfix/main.cf:
# The non-logging alternative:
smtpd_discard_ehlo_keywords = chunking, silent_discard
Trying this did not work, "discarding...CHUNKING" still appeared in
logs.
I then checked the documentation on the setting itself at
http://www.postfix.org/postconf.5.html#smtpd_discard_ehlo_keywords
which says "Specify the silent-discard pseudo keyword to prevent this
action from being logged."
I changed "silent_discard" to "silent-discard" in main.cf and it stopped
logging the CHUNKING messages.
/etc/postfix/main.cf:
smtpd_discard_ehlo_keywords = chunking, silent-discard
