No IDNA Support In Lotus Notes?

It looks like Lotus Notes does not support Internationalized Domain Names. If you want to send an email to somebody where the domainname has e.g. german umlauts you have to use a punycode encoder tool, replace the domainname with the encoded domainname string and then send it.
Otherwise the mailrouter gives you a ‘no route found to domain…’ failure.
[RFC 3492]
Online converter:
IDN Online Converter (german)