[exim] How to handle UTF8 domain names from (stupid?) client…

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Author: Heiko Schlittermann
To: exim-users
Subject: [exim] How to handle UTF8 domain names from (stupid?) clients?

we've some clients sending messages to umlaut (UTF8) domains. As far as I know,
the client is responsible for converting any UTF8 domain into the punycode

It seems, that there are clients (squirrel mail, thunderbird, …?) not
beeing aware of the problem.

As far as I see, there are "allow_utf8_domains", and
"dns_check_names_pattern" which can influence Exims handling of UTF8
domains, but finally Exim uses the local resolver for resolving the

I'm thinking about using a rewrite rule to transfrom the UTF8 domain names
into punycode ones.

Without rewriting the domains Exim would ask the resolver about the UTF8
names. Even if the resolver would resolve such names, I think, there is
no guarantee about how other MTAs will handle such message.

My questions:

Are there other or cleaner solutions out there?
Does somebody already do it?
Does there an UTF8 capable DNS resolver exist?

Thank you and Greetings from Dresden/Germany,
Heiko Schlittermann