I need configure the server to relay to an external host. For testing I am using imapsync to pull from external IMAP server to the Zimbra test machine, there need use 'PLAIN' as authentication method as that is what the mail server supports ONLY. No SSL, no nothing. It feels like I am just missing something, been at this trying over and over different options. Last time I did something similar was on Suse Enterprise server, had to configure Postfix to relay to my mail server. So I know (at least at that time) this hosting service allowed for relaying so long as I was authenticated.

I followed the Wiki article and ran the following commands;
zmprov mcf zimbraMtaRelayHost smtp.MYDOMAIN.com:587
echo smtp.sbn-services.com sam@MYDOMAIN:PASSWORD > /opt/zimbra/conf/relay_password
postmap hash:/opt/zimbra/conf/relay_password
postmap -q smtp.MYDOMAIN.com /opt/zimbra/conf/relay_password
zmlocalconfig -e postfix_smtp_sasl_password_maps=hash:/opt/zimbra/conf/relay_password
zmlocalconfig -e postfix_smtp_sasl_auth_enable=no
zmlocalconfig -e postfix_smtp_cname_overrides_servername=no
postfix reload

Right now my main.cf Postfix file has;
smtp_sasl_security_options = PLAIN
relayhost = smtp.MYDOMAIN.com:587
smtpd_sasl_authenticated_header = no
smtpd_sasl_security_options = noanonymous
smtpd_recipient_restrictions = reject_non_fqdn_recipient, permit_mynetworks, reject_unlisted_recipient, reject_invalid_helo_hostname, reject_non_fqdn_sender, permit
smtpd_relay_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination
smtpd_reject_unlisted_recipient = no
smtp_sasl_mechanism_filter = plain,login
smtpd_tls_auth_only = yes
smtpd_sasl_auth_enable = yes
smtpd_tls_loglevel = 1
smtp_tls_security_level =
smtp_sasl_password_maps = hash:/opt/zimbra/conf/relay_password

And I get this for log result;
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 220 ESMTP Fri, 26 Jul 2013 02:36:29 -0400: UCE strictly prohibited
Jul 26 01:36:27 zimbra postfix/smtp[26976]: > smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: EHLO zimbra.MY-DOMAIN.com
Jul 26 01:36:27 zimbra postfix/smtp[26976]: vstream_fflush_some: fd 14 flush 30
Jul 26 01:36:27 zimbra postfix/smtp[26976]: vstream_buf_get_ready: fd 14 got 156
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250-bosauthsmtp01.eigbox.net Hello zimbra.MY-DOMAIN.com [76.xx.xxx.xx]
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250-SIZE 34603008
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250-PIPELINING
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250-AUTH PLAIN LOGIN
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250-STARTTLS
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 250 HELP
Jul 26 01:36:27 zimbra postfix/smtp[26976]: server features: 0x101d size 34603008
Jul 26 01:36:27 zimbra postfix/smtp[26976]: Using ESMTP PIPELINING, TCP send buffer size is 16384, PIPELINING buffer size is 4096
Jul 26 01:36:27 zimbra postfix/smtp[26976]: smtp_stream_setup: maxtime=300 enable_deadline=0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: > smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: MAIL FROM:<sam@MY-DOMAIN.com> SIZE=1553
Jul 26 01:36:27 zimbra postfix/smtp[26976]: > smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: RCPT TO:<******@gmail.com>
Jul 26 01:36:27 zimbra postfix/smtp[26976]: > smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: DATA
Jul 26 01:36:27 zimbra postfix/smtp[26976]: smtp_stream_setup: maxtime=300 enable_deadline=0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: vstream_fflush_some: fd 14 flush 80
Jul 26 01:36:27 zimbra postfix/smtp[26976]: vstream_buf_get_ready: fd 14 got 77
Jul 26 01:36:27 zimbra postfix/smtp[26976]: < smtp.MY-DOMAIN.com[66.xx.xxx.xx]:587: 550 bosauthsmtp01: Host 76.xx.xxx.xx: No unauthenticated relaying permitted
Jul 26 01:36:27 zimbra postfix/smtp[26976]: connect to subsystem private/bounce
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr nrequest = 0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr flags = 0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr queue_id = 87986DA1F2
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr original_recipient = ******@gmail.com
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr recipient = ******@gmail.com
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr offset = 602
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr dsn_orig_rcpt = rfc822;******@gmail.com
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr notify_flags = 0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr status = 5.0.0
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr diag_type = smtp
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr diag_text = 550 bosauthsmtp01: Host 76.xx.xxx.xx: No unauthenticated relaying permitted
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr mta_type = dns
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr mta_mname = smtp.MY-DOMAIN.com
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr action = failed
Jul 26 01:36:27 zimbra postfix/smtp[26976]: send attr reason = host smtp.MY-DOMAIN.com[66.xx.xxx.xx] said: 550 bosauthsmtp01: Host 76.xx.xxx.xx: No unauthenticated relaying permitted (in reply to MAIL FROM command)