@lanodan @ada I checked RFC 5321 and there should only be one or three colons after IPv6 (i.e. either [IPv6:0:0:…] or [IPv6:::1]). The initial colon is separator of the address type and actual address.
Then my Postfix accepts it says there is no such user (obviously), before it was bad syntax.
the best is to try sending
Does anyone remember the VRFY command?