Hi There,
I had the same problem. But i managed to get around because i work for an internet company.
The main problem is that the registration email comes from a domain that doesn't actually exist (or a sub-domain, i cant remember exactly).
Our mailserver rejects the email because it's "from" address is from a non existent domain (as an anti-spam measure). I got around this by setting up a fake entry on my DNS servers, so that my mailserver would accept the email ! I imagine the problems that others (such as yourself) are getting may be the same.
That's the technical reason! Sorry if it's a bit "techy", but not sure how else to explain it. I meant to raise it with the forum admin, but never got around to it. They may be aware of this already.
I'd suggest either it needs to be changed to come from a valid domain/sub-domain. You could try the same trick as me, but your ISP is very unlikely to help you with that. I'm just very lucky as I have the tools here.
Andy