Umlaut domains and IDNPunycode, email limits and when it is worth it

Published on October 20, 2026

What an IDN is

IDN stands for Internationalized Domain Name - a domain with characters outside the old standard of letters, digits and hyphens. For .de, umlaut domains have been registrable since 1 March 2004.

For .de, besides ä, ö, ü and ß, numerous other diacritical characters of European languages are permitted. Not permitted are emojis, spaces and characters from different writing systems in the same domain.

Punycode: what happens technically

DNS itself knows no umlauts. That is why every IDN is translated into a pure ASCII form before resolution - the Punycode, recognizable by the prefix xn--:

The browser handles the conversion automatically. You register and manage the domain in the readable spelling; in certificates, server logs and some tools, however, the xn-- form appears. That is normal and not an error.

Where umlaut domains work reliably

• Browsers: all current browsers resolve IDNs correctly and display them readably.
• DNS management: you set records as usual, see DNS records explained.
• SSL certificates: are issued; the certificate contains the Punycode form.
• Search engines: index IDNs without disadvantage compared with ASCII domains.

Where it gets stuck

• Email addresses. The biggest catch: addresses such as info@müller.de are not processed cleanly by many mail systems. The local part before the @ is not intended for umlauts outside SMTPUTF8 anyway, and support is patchy across the landscape. Setting up email on your domain is covered in Use a domain for email.
• Passing it on verbally. "Müller with ü or ue?" - exactly the follow-up question that makes an address impractical.
• Older software. Legacy systems, print templates and some form validations cannot cope with xn--.
• Risk of confusion. Characters from other writing systems can look deceptively similar to Latin letters. That is why browsers display the Punycode as a precaution for mixed scripts.

The practical recommendation

In most cases, the combination is the right solution:

1. Spelled-out variant as the main domain - mueller.de. Website and email run on it.
2. Also register the umlaut domain - müller.de. It catches those who intuitively type with an umlaut.
3. Redirect via 301 to the main domain so that no duplicate content arises - see .htaccess redirects.

The other way round - the umlaut domain as the main address - is only worth it if the name becomes really unreadable without the umlaut and you do not need email addresses under it. More general naming tips are in Find the right domain name.

What happens with ß

A special case with a history: the German ß was initially treated automatically as ss for .de. Since November 2010, it has been a character in its own right and can be registered separately.

So straße.de, strasse.de and strasze.de are three different domains that can belong to different owners. If your name contains a ß, secure both common spellings.

Frequently asked questions