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You should find the following lines in the /opt/zimbra/postfix/conf/master.cf:
Originally Posted by ralph
uncomment them (maintaining the white space at the beginning of lines two & three) then restart postfix. You'll need to make this modification after each upgrade of Zimbra. There's also an RFE in bugzilla for this feature to persist and be enabled by default, search and vote for it.
#submission inet n - n - - smtpd
# -o smtpd_etrn_restrictions=reject
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject
You should also upgrade to the most recent ZImbra release.