When opening an Internationalized domain name (IDN) in your browser (chrome, firefox, safari, etc.) the domain is converted to the so called punycode. So that the domain can be prcessed technically.
converted to: xn--mller-kva.de
Unfortunately some programs (mostly older) do not support this conversion yet. Which means especially for sending e-mails to adresses with mutated vowels.
You will find an explanation for Internationalized domain names (IDN) under the follwing link:
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