This is what I have found in /var/log/zimbra.log
Jan 29 09:23:34 mail postfix/smtpd[25202]: connect from mail.coqueths.org[217.23.230.158]
Jan 29 09:23:34 mail postfix/smtpd[25202]: NOQUEUE: reject: RCPT from mail.coqueths.org[217.23.230.158]: 554 5.7.1 <USER@googlemail.com>: Relay access denied; from=<adminkc@coqueths.org> to=<USER@googlemail.com> proto=ESMTP helo=<mail.coqueths.internal>
Jan 29 09:23:34 mail postfix/smtpd[25202]: disconnect from mail.coqueths.org[217.23.230.158]
I have taken out the username from the googlemail address.