Use sasl rimap (remote imap) for authentication and authenticate against the zarafa imap gateway.

Install sasl2-bin

apt-get install sasl2-bin

create /etc/postfix/sasl/smtpd.conf

pwcheck_method: saslauthd
mech_list: plain login

First off postfix appeared to need to be a member of the sasl group

gpasswd -a postfix sasl

This needed setting in the postfix main.cf

smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,check_relay_domains,reject_unauth_destination

the complete working /etc/default/saslauthd

START=yes
DESC="SASL Authentication Daemon"
NAME="saslauthd"
MECHANISMS="rimap"
MECH_OPTIONS="127.0.0.1"
THREADS=0
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd -r"