SMTPUTF8 support

[Daniel Lange]

Am 05.04.19 um 22:36 schrieb David Bürgin via dovecot:
> I’m afraid there is a possibility that your setup only works by chance …
> I don’t think Dovecot LMTP is SMTPUTF8-capable according to RFC 6531. It
> needs to advertise the SMTPUTF8 extension but doesn’t.
[..]

Dovecot doesn't know anything about SMTPUTF8 as that has not been 
implemented in either LDA or LMTP as of now.

Worse, it will create bounces on postfix accepted UTF8 localpart 
addresses as per
https://www.dovecot.org/pipermail/dovecot/2019-March/115286.html

This one is tracked as DOP-1045 as per Aki's email linked above.

No idea when SMTPUTF8 will be implemented. It will create a big mess as 
Postfix with smtputf8=on defaults are being used _and_ people start 
using UTF8 localparts.
Cf. http://www.postfix.org/COMPATIBILITY_README.html#smtputf8_enable
for postfix info.

You should configure a Postfix that delivers to Dovecot with
smtputf8_enable=no until Dovecot learns SMTPUTF8.
But even in such a defensive setup Dovecot LDA and LMTP can still create 
backscatter spam bounces as explained in the mailinglist thread linked 
above.