Howdy, I'll create my own thread to get this known as well as updating the wiki, but 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.
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 per the wiki page:
Outgoing SMTP Authentication - Zimbra :: Wiki