[exim] Why are some messages just being queued?

I am running exim-4.50.

I am running an exim server to handle routing of mail submitted by my users (20,000
of them). they are either sent out over the Internet or passed to our IMAP/POP
server for local delivery (both by SMTP).

I have recently noticed that sometimes exim is just queueing messages. I have
noticed this with my own messages (both remote and local delivery). There is no
indication in the log as to why (I have the delay_delivery log selector on by
default). I can kick the delivery using eximon, so the exim process which handled
receipt has finished with the message. Most messages are processed on the fly.

There is nothing unusual about the messages; they have a single recipient and are
small. At the times when I have seen this the load average has been very low and
the number of exim processes has been well below smtp_accept_max, etc. The only
queue-related option I can see is smtp_accept_queue_per_connection=10, which is not
applicable with these messages.

Any ideas? Can I get exim to log a reason?

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