hmm.. you could probably make yabbse accounts work with an external free email service, the problem with that is that you have to change your MX (was that the name?) record for the domain and you won't be able to use your normal email anymore, only the one they service
hosting the email accounts yourself will cost you a lot, because of the bandwidth and hdd used, but you could just setup email redirections, which would cost you nothing... for something like this you'd need a webhost with something like cpanel, that gives you control over that kind of things
I don't know if any of these two can be combined with the yabbse registration process though, but I thought I'd mention it
