> What I do not understand, postfix-wise, is that I have seen no warnings
> about "using backwards-compatible" default value of smtputf8_enable
> during the period where I was using compatibility_level=0.?
With compatibility_level=0, Postfix always accepts mail with a
non-ascii header or address localpart. Whereas compatibility is
easy to check for features that are implemented in one place,
smtputf8 affects a lot of programs. One would have to enable it
under the covers, but not enforce it.