[Mailman-Users] Support for Internationalized Domain Names and Email Addresses

Stephen J. Turnbull turnbull.stephen.fw at u.tsukuba.ac.jp
Thu Jul 21 03:39:42 EDT 2016

Russ Housley writes:

 > My add addresses that include IDN or EAI have not worked.
 > Subscriptions are rejected.

If you are using the wire-format of IDN addresses, ie, after being
encoded to A-labels (ASCII characters in the IDN form), then it should
work: those are valid mailboxes as ASCII.

If you're using EAI, lists are (generally speaking) problematic.  EAI
requires the SMTPUTF8 extension to SMTP in *all* relays involved.  Are
you sure all your subscribers are SMTPUTF8-capable?  (That's a real
question, I can easily imagine you're in a situation where you do
know.)  If not, an EAI address will likely DOS those that aren't.
Note that it's not unlikely in most contexts that some of them forward
to systems which don't support SMTPUTF8.

What can Mailman do?

Unfortunately, implementing EAI in Mailman is going to require a lot
of thought for Mailman development and for site administrators, since
enabling SMTPUTF8 in the MTA means that EAI subscribers will likely
*send* messages from their EAI addresses.  That means that those
messages will *hard bounce* at recipient sites (or their secondary
MXes) that do not support SMTPUTF8, and you'll be DMARCing[1] those
subscribers into oblivion -- they simply won't receive that mail, and
maybe they even get unsubscribed (I suppose that the bounce will have
a special status code which Mailman can recognize, though).  Unless
Mailman's host MTA is smart enough to DTRT (but what's that?) with the
EAI address and resend.  But I don't want to think about what resending
means for things like DMARC,[2] and IIRC the EAI RFC did a lot of
handwaving about intermediate sites -- and assumed they were pure
"next hop" forwarders, not mailing lists, in that discussion.

Mailman also is installed at a lot of sites where the default encoding
is not UTF-8.  This probably isn't a problem (it may even be a
non-problem), but it needs consideration in dealing with web interface
and error messages.

I also worry about whether the big poorly administered domains like
AOL and Yahoo![3] will treat such addresses as a spam signal.  So far
*all* of the mail I've received with raw UTF-8 in the header has been
spam.  (I'm not judging EAI users, of course, I'm simply remarking on
a particular sample with distressing implications if it turns out to
be at all representative.)

[1]  The analogy is quite exact -- certain sender addresses cause
innocent subscribers to bounce the post.

[2]  Theoretically there are a couple of proposed protocols that might
help, but we have no idea whether there will be any uptake of them.
The past behavior of Yahoo!, Hotmail, and AOL suggests pessimism is in

[3]  Usually not the technicians' fault, but rather management that
prefers losing mail to delivering spam ever.

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