[exim] spec about hyphens in host names

Hello,

In 14.23 of the spec in the explanation of dns_check_names_pattern
there is after "which permits only letters, digits, slashes, and hyphens
in components, but they must start and end with a letter or digit." the sentence
"
Hyphens are not, in fact, permitted in host names, but they are found in
certain NS records (which can be accessed in Exim by using a dnsdb lookup).
"

Any reference for hyphens not beeing allowed in host names?
I cannot find any, see e.g. the discussion at
http://www.imc.org/idn/mail-archive/msg04905.html .
Maybe this was mixed up with underscore?

Greetings,
Lutz

-- 
Lutz Preßler  <Lutz.Pressler@???>    http://www.SerNet.DE/
SerNet Service Network GmbH, Bahnhofsallee 1b, D-37081 Göttingen
Tel.: +49-551-370000-2,      FAX: +49-551-370000-9
AG Göttingen, HRB 2816,      GF: Dr. Johannes Loxen