upgrade postfix configurations to stretch/buster
our postfix configuration as delivered by puppet is pretty old, probably pre-stretch. it's generally not a problem until some upgrade breaks certain assertions. buster seems to have at least broken one, as described in #31743 (moved), so it seems it would be a good idea to upgrade our configs everywhere.
first I'll look on all hosts to see if we have this problem elsewhere and then I'll cleanup the configs to port to the brave new world...
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