Subject: Re: [Uta] SMTP Over TLS on Port 26 - Implicit TLS Proposal
From: "John Levine" <[email-masked]>
To: [email-masked]
Cc: [email-masked]
Date: 5 Jan 2019 20:08:28 -0500
List-Id: UTA working group mailing list <uta.ietf.org>

In article <CAOEezJTyEf+Sn9ZqQPue1DFUSoFO211YogJ6ufYJxswWzXk=[email-masked]> you write:
>> For one thing, it doesn't work with IDNs.

>I just tested "26pref-español.mydomain.com
><http://xn--26pref-espaol-skb.mydomain.com>" in this page
><https://www.verisign.com/en_US/channel-resources/domain-registry-products/idn/idn-conversion-tool/index.xhtml?loc=en_US>
>
>The output looks like "xn--26pref-espaol-skb.mydomain.com". So instead of
>"if string starts with", we should go for "if string contains".

IDNs use a complex encoding and there is no reason to assume the encoding
won't occasionally include the string "26pref".

Really, this is what SRV is for.  It's not hard to program, and we all
know that mail is not the web and the time for DNS lookups (of which
there are already at least two, more if there's multiple MX) is not in
the critical path.  If you want to be clever, do the SRV and MX
lookups at the same time.