[exim] timing for large numbers of recipients

One of my servers (exim-4.41) expands distribution lists. Local recipients are
passed to another exim-4.41 server for delivery to my cyrus POP/IMAP server using
a LMTP transport (using socket).

I am concerned about handling 20,000 users in one go. cyrus uses hard links for
multiple recipients to save filespace, so I want to batch as much as possible. I
have set "rcpt_max = 25000" on the SMTP transport on the list server so that it does
not split the message.

However, if I try to deliver all 20,000 in one go on the cyrus server then cyrus
takes so long that exim times out the LMTP session. That results in exim re-trying
and cyrus re-delivering to the same users before the timeout triggers again. It
would need a very large and rather unpredictable timeout value, so I need to use
smaller batches.

I have set "batch_max = 1000" on the LMTP transport.

Will the SMTP transport sending from the list server to the cyrus server time out?
As I understand it, exim normally delivers "on the fly" when a message is received
by SMTP for local delivery (LMTP appears to be a local transport). Does batch_max
alter that so that the SMTP session closes straight after the DATA phase or does the
session wait until all deliveries have completed?

Perhaps I need to set queue_only in a data ACL when there is a large number of
recipients.

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