Howdy, I updated the wiki, but wanted to put out a forum post to get indexed as well so someone else saves some time. Specifically AT&T Yahoo DSL (consumer at the very least) doesn't implement TLS properly on the advertised port (465) if you use port 587 it works properly as per the wiki.
This leads to this error in /var/log/mail.log when the "vanilla" wiki page is followed:
lost connection with external.relay.com while receiving the initial server greeting
Wiki page: Outgoing SMTP Authentication - Zimbra :: Wiki
Hope this saves someone the six hours it took me to dig up on an ubuntu forum unrelated post.
postfix, SSL and AT&T [Archive] - Ubuntu Forums
Specifically the post by mterlouw on August 19th, 2007, 04:47 AM.
This is confirmed to work with Zimbra on my consumer dsl connection.