Any idea why mltbd is appended to your domains? It seems a little proprietary. The RACE encoding of hørup.com is bq--abupq4tvoa.com so there should be NS records for bq--abupq4tvoa.com but there isn't. Does Verisign simply create all IDN's as subdomains to mltbd.com?
If this is correct all registrars could create an IDN plugin for their browser and start to create IDN's in their own namespace and still claim to be compliant with the IDN specification created by the IETF
I have no idea why they have done it this way. It can be a proprietary way of solving a problem, that should have been solved in the DNS, but until the Internet Standard for Internationalized Domain Names has been finally approved and then later implemented into all DNS (which could take 5-10 years untill all is updated), we do need a in-between solution.