pfSense - How to add a MX record to a local zone

My plan was to have a Zimbra e-mail server running in VirtualBox using the internal networking model with a local domain. To make this work, I have a virtual pfSense box running on the same internal network to provide “outside” access. Additionally, pfSense is also running a DNS resolver so I figured it would be a trivial matter to add the local records I needed.