To allow for flexibility, redundancy, and abstraction and to provide a layer of security, organizations often utilize a concept known as proxy servers. They are fairly standard within the web server side of the infrastructure, but less so for messaging.
The Sun ONE Messaging Server software environment offers two proxy servers:
messaging multiplexer proxy (MMP)
messenger express multiplexer (MEM)
MMP proxies POP and IMAP connections, whereas the MEM off-loads the web mail client from the mailstore, so in a true sense MEM is a front-end, not a proxy.
Why a proxy function? Well, in a large Internet Service Provider or in environments that have grown beyond a single mailstore, the proxy hides the fact you have multiple back-end servers, ...