Edit /opt/zimbra/maps/postfix_black_white_list

Add the following entries

# a comment
5.5.5.5      REJECT Stop sending me spam
4.4.4.0/24   REJECT All your servers are compromised
fubar.com    REJECT I want no email from anyone@fubar.com
.fubar.com   REJECT I dont want email from your subdomains either
goodguys.com OK  I want all your email
1.1.1.1      OK  I like your server
2.2.2.0/24   OK  Your server farm is ok

execute the following:

/opt/zimbra/postfix/sbin/postmap /opt/zimbra/maps/postfix_black_white_list 
su zimbra
zmprov ms `zmhostname` +zimbraMtaRestriction 'check_client_access lmdb:/opt/zimbra/maps/postfix_black_white_list'
postfix reload

To maintain the list, just edit /opt/zimbra/maps/postfix_black_white_list  then execute

/opt/zimbra/postfix/sbin/postmap /opt/zimbra/maps/postfix_black_white_list