Seems to be a solid release. I did have to execute the following to send/receive after the upgrade:

zmlocalconfig -e postfix_transport_maps=' hash:/opt/zimbra/postfix/conf/transport,ldap:/opt/zimbra/conf/ldap-transport.cf'

postmap /opt/zimbra/postfix/conf/transport

chown root:root /opt/zimbra/postfix-2.7.4.2z/conf/ -R

zmmtactl stop
zmmtactl start