I have been beating my head for the last two nights trying to get SMTP authentication running from my house to my server at another location.
I ended up fixing it be removing "fixup esmtp" from the Cisco Pix (ASA) configuration.
My Zimbra server sits on a DMZ interface off the Pix. It is not directly connected to the Internet
As soon as I did that, I could send using SMTP authentication on both Outlook Express and SeaMoney's email client...
That was my last little thing I needed to have everything running...
These forums have been great help!