Every once in a while I get a message from a user about some error he/she got while trying to send an email. They forward me the error message but it is usually too vague to understand what the problem is. Two cases come to mind: the account doesn't exist on the foreign server or it is over quota. If the user could see the smtp error code (and message), they could understand (at least some of them) what the problem is and correct it themselves.
So my suggestion is that the Zimbra Connector for Outlook includes the original smtp error code and message in its error message to the user.