We are experimenting with using Zimbra as an internal email server. We only allow outbound email to our external email server, and to text messaging addresses for automated notification. I've set up the allowed sender and recipient domains using the following info:

Restrict sending to certain domains - Zimbra :: Wiki

This works perfectly. If a user is using the zimbra client, sending email to a restricted domain is instantly rejected and the user gets that message.

I also tested thinks by actually telnetting into the zimbra server using this advice:

RestrictPostfixRecipients - Zimbra :: Wiki

Again things work as expected.

The problem is that I have some users connecting using Outlook 2010 via IMAP to the zimbra server. These same users are using Outlook 2010 to connect to our external corporate email server as well. When they mistakenly send emails using their "internal" restricted account, the email is rejected as expected, but the user gets no notification whatsover. The email appears in their sent folder in Outlook 2010 as if it had been sent. This causes confusion as the users don't realize their mistake in a timely way.

My question - is there some way to configure zimbra so that it will instantly send a rejected email notice back to the sender in Outlook 2010, similar to the experience in the zimbra client?