[exim] Return temporary error after DATA but store mail?

Hello,

is it possible to store mail to a file or route it otherwise, but to
return a temporary error after DATA? I am curious what kind of spam will
be delivered to a backup MX, if the primary MX is up, but I don't want
that host to actually be a backup MX in case the primary host fails.

I am not sure Exim can do that. The closest I found was

control = fakereject

but I need something like "fakedefer".

Michael