as has been mentioned, this does not work too well when interacting with
the "variables" extension. the generated message will often contain the
subject of the original message etc., and therefore be unique. I
suggest
Vacation responses are not just per address, but are per address and
reason argument.
I think that makes it clearer that the intent is to consider the
verbatim unexpanded reason argument.
For correctness then:
Vacation responses are not just per address, but are per address, ":mime"
option and reason argument.
Michael