I don't think you can  Can you configure the MFP to send to a specific MTA port ? If so you could run a second instance of Postfix which will only accept mail from the IP address of the MFP, and then have forward to the standard SMTP port for Zimbra.
__________________ Server | CentOS 5.1 | Dual Opteron 250 | Tyan K8W Mobo | 6GB RAM | 3WARE 9550-SX4 | 4 x Samsung 200GB SATA II | Zimbra | Release Release 5.0.7_GA_2450.RHEL5_20080630192737 NETWORK edition running on Xen 3.2 CentOS 5.2 i386 VM | Network | Cisco 877 Router - Cisco ASA 5505 FW - Cisco 1131AP | |