[exim] exim 'problem'?

I have exim with the exiscan patch running on FreeBSD 5.2.1-p8, box has
been up 157 days doing great (exim is from ports).

About two months ago we put exim into 'daemontools', running great not a
problem at all. A few days ago (monday) we decided to try running with
the exim spool file in a freebsd mfs (memory file system a.k.a.
ram-disk). For all our testing it seemed fine and fast. We made a 64mb
mfs mounted it at /var/spool/exim and changed the perms to be
mailnull:mail. When we started exim up it recreated the directory
structure like it should have, and as I said all ours tests were great.

So on tuesday we turned exim down and mounted the mfs on the main server
and restarted exim, and we monitored it closely for two days, again not
a single problem.

Well yesterday (thursday afternoon) there was a strange thing that
happened with exim, it delivered the same inbound email 8-25x each. So
people got multiple copies.

When we got complaints from people the only thing we could think of was
to remove exim from the mfs, after doing that the problem seemed to go
away..

So my question is, is exim mounted on a mfs partition not supported?
Are there some other options that we would need to set either in exim or
FreeBSD to make this working?

Has anyone gotten this to work? Is this an unneeded 'change' that we made?