bug#21733: 25.0.50; Umlaut in address makes smtpmail-via-smtp fail

[Random832]

Stephen Berman <stephen.berman@gmx.net> writes:
> I tried to send an email with Gnus to an address containing an umlaut
> ('ä') in the domain name, and it failed with the error "Sending failed:
> 501 Syntax error in parameters or arguments".  When I tried sending the
> same email to the same address using the GMX webmail interface, it
> succeeded.  A copy of the sent email showed an address containing only
> ascii characters instead of the address with the umlaut, so I assume the
> latter is aliased to the former and GMX automatically found this.

Did this address begin with "xn--" in the segment which had contained
non-ascii characters in the original, such as "xn--nt-via" for "nät"?

> It
> would be nice if smtpmail.el could do the same thing instead of erroring
> out.  I can reproduce the same error using the fake address
> <stephen.berman@gmx.nät> (backtrace attached).

There is a file called punycode.el in libidn which encodes strings in
the way I mentioned above, maybe it would be worthwhile to include this
in Emacs - or at least have a hook which will use it if possible and
error out immediately (for domains containing non-ASCII characters) if
it's not present. This would be relevant to more places than just email.

This uses a command-line tool (part of libidn) to do the actual
decoding, I don't know how feasible it would be to do it in pure elisp.