[Exim] headers_add lost if deferred

I would be surprised if this is a bug which has not beed picked up before, so I
guess I must be missing something. I have found added headers to be dropped if a
delivery defers.

I have an accept router which has a headers_add line. It routes to an appendfile
transport. The transport has a "mode = 0660" line. If the file being appended to
has a mode of 0660, all is well and the message is delivered with the router's
header line added.

If, however, I change the mode on the file to force the delivery to defer, then
change the mode back to 0660 and use eximon to re-try the delivery, I find that the
router's header is not in the message which is delivered!

I am using a test configuration and submitting the message with:

exim -C test-config a.n.other
From: me@here
To: a.n.other
Subject: test

test
.

Any ideas?

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