The wiki article describes
SMTP over SSL and is (IIRC correctly) enabled by default in Zimbra 5, the submissions port 587 is not enabled by default. You should have a look in /opt/zimbra/postfix/conf/master.cf for the following lines:
Code:
#submission inet n - n - - smtpd
# -o smtpd_etrn_restrictions=reject
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject
and uncomment them, the white space in front of the second and third lines is important and must be there.
Modify your Zimbra server to require authentication and restart it and you should be set. The port 587 will not survive an upgrade and you'll need to modify that after you do any Zimbra upgrade, there's an RFE in
bugzilla to make that change permanent if you want to vote on it.