Apart from checking over your DNS MX records manually (and double checking the guide), I’m not sure. I’ve only set the mail-receiver up a handful of times, and it’s been pretty plain-sailing every time (not to rub it in). When I check the mx:community.mountainbikemuseum.net mx record it says its not the fully qualified domain name, so I can’t tell if the mx records are pointing to the right place. (Edit: I put a www. in front 
It’s inconclusive, but looks similar to my working one. I’ve sent a test email to it just on the offchance)
@IAmGav may be on to something that I can’t spot, so hopefully he can give you a bit more detail about those steps.
Just to confirm, you’re the OP with another account?
I’ll throw in a link to the Troubleshooting section for good measure. 