A little research indicates that using many "freedom boxes" (plug computers) in a cluster for running mail hosting services is probably unfeasible or impractical. Both cost and watts per mhz would presumably be poor, especially considering that the ARM CPUs are only 32 bit.
If a single freedom box would be sufficiently powerful to constitute the server, then that remains an option (I can't find much conclusive data on this, but I gather that it may be not be the case if we attempt to host both webmail applications (eg roundcube) and also mail server on the same device).
An alternative would be to use the fit-PC3 (see http://www.fit-pc.com/web/purchase/order-direct-fit-pc3/). This a new product with the following advantages:
64-bit dual core 1ghz processors many standard CPU extensions, inc. virtualisation (http://s.coop/7quk) 2GB DDR3 RAM *only 9 watts power consumption*
It is quite expensive however at 454 USD (292 GBP) inc. shipping from the states. We would need to get a hard drive + caddy and cable separately (approx. 80 gbp more, without a backup drive). If we had funding of some sort then currently this machine would definitely be my choice.
I have yet to find out if there is second hand hardware that we could use from Arcspace. I intend to do so shortly.
Thanks!
Sam.