I would like to use Zimbra's postfix as an unauthenticated outgoing SMTP relay for mail originating from localhost and all RFC1918 addresses in the Zimbra server's subnet. I happen to like the mutt MUA so I'm using it on localhost as the testing client. This applies only to mail sent from a MUA besides Zimbra's web interface, which is lovely...but it's all about choice, right? Anyhoo, Here's where I'm stuck.

Open mutt with
mutt -f imaps://localhost/INBOX
then login as normal. View messages in inbox. Create new message to address external from Zimbra domain. Set From: user@zimbra.domain header in mutt. Send message. mutt reports message is sent successfully. Message never reaches it's destination. There is no bounce. It's gone. Running
says queue is empty, so the message must have gone somewhere. But where?