How to find the host name 

host `hostname`

Domain level blocking of users

ZCS8:
zmlocalconfig -e postfix_smtpd_sender_restrictions=”hash:/opt/zimbra/postfix/conf/reject”

ZCS8:
Add hash:/opt/zimbra/postfix/conf/reject as the first line of /opt/zimbra/conf/zmconfigd/smtpd_sender_restrictions.cf

user@domain.com REJECT
domainX.com REJECT

Restart zimbra
postmap /opt/zimbra/postfix/conf/reject
zmmtactl stop && zmmtactl start