[Exim] max_batch and lmtp driver

I have been looking at the cyrus imap/pop server package with a view to moving our
service to use it. One of the attractions of cyrus is that is only holds one copy
of a message when it delivers to multiple recipients when using LMTP.

i.e. using a router and transport as follows:

cyrustest:
driver = accept
local_parts = test1:test2:test3
transport = cyrus

cyrus:
driver = lmtp
command = "/.../bin/deliver -l"
user = cyrus
batch_max = 20

We have never allowed the sending of a single message to every user with the
existing standard local delivery transport because of the filespace overhead of
20,000 copies being generated. If we used cyrus then that should no longer be a
problem.

My question is, can I set "batch_max = 20000"? Or, I guess I should ask, what would
be the consequences of doing that? Would it result in a massive demand on memory or
some other finite resource?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter