Pinjo can easily be used to be the frontend for multiple mailservers. The distributing mechanism sends all mail to the correct mailserver.

Splitting of email is done on basis of domain. Email for *@servera.com is sent to server A, and email for *@serverb.com is sent to server B.. etc etc. The number of domains is not limited.
In case of multiple domains in 1 message, Pinjo even splits up the message and delivers them to the right servers.