[exim] Features for Exim 4.85

Hi,

The Exim developers are now starting to consider what
currently-experimental features should be moved to the
mainline (meaning, the default Makefile includes them)
for version 4.85.

One proposal is EXPERIMENTAL_DSN. Could people using
it currently please report their experience and any
(or none) issues?

Any others people are interested in, likewise.
--
Thanks,
Jeremy