Oh joy, the missing Pi has arrived - RS had delivered 9 instead of the 10 that they should have, and today the additional one arrived.
I opened the box, and without opening the sealed transparent envelope containing the Pi, I could see there was a problem. The large capacitor (the same one that is loose on the faulty Pi that they said they'll replace) was rolling around inside, completely unattached. So I'll have to send this one back also.
Meanwhile, I've got the two Pies that will be acting as twin mailservers ready. You ought to have two mailservers, so that if one of them fails, the other one is still accepting email. I'll be putting them in place tomorrow, replacing the two full-sized computers (one of which had a flaky hard drive).
And next, I'll bring up two more mail servers, plus I'm going to see if I can use a Cisco Pix for the DMZ firewall (I'm currently using a Sonicwall, and the dratted thing only allows 20 firewall rules, which is rather a limitation). I have two Pixes which are currently doing nothing, and anyway I can get a Pix on Ebay for a mere £50 (they're worth £500). The reason they're so cheap on Ebay, is that corporates won't buy second-hand kit, and home users don't use Pixes.
It occurs to me that I could use a Pi as a firewall; all it would need is a second ethernet port (running off a USB port) and some software that I've already written (I was running a homebrew firewall for quite a while).
I've found that an unregulated power supply that will output 1500 ma at 6v, is fine for powering three Pies. I'll try it with four, but I'm planning to use each one to power only two. I found that, with no load,the power supply consumes 5 watts, and each Pi adds another 5 watts to the load. That means, 2.5 watts for the Pi, and 2.5 watts in efficiency losses. but that such a low level of current draw, efficiency losses aren't a big deal.