I suppose you like to set up SMTP authentication on your mail server, but your Postfix configuration does not show any authentication configuration for clients. You could do this using SASL against the Zarafa-gateway (IMAP) like described here. Then configure Postfix to use your SASL (Cyrus for example) and 'trust' authenticated clients. Read more about that in the Postfix SASL Howto (part 'Mail relay authorization' in particular).
This way, all IMAP enabled users in Zarafa will have authorized SMTP relay access on your MTA.