[Exim] failure to transfer data

I have an entry in the paniclog which I can't work out. It says:

<date-time> <message-id> internal problem in user_forward router (recipient is
xyuser@???): failure to transfer data from subprocess

The user_forward router is:

user_forward:
driver = redirect
domain = <my domains>
caseful_local_part
check_ancestor
check_local_user
no_expn
file = $home/.forward
address_data = by_user_forward
file_transport = address_file
pipe_transport = address_pipe
reply_transport = address_reply
no_verify

Now this router works fine for loads of other users who have .forward files set up.
It is just this one user which is causing the problem.

I have checked the home directory and the permissions/ownership of the directories
and files and they are all OK. The .forward file contains

xyuser@???

I find that s-devon.ac.uk is not in the DNS, but surely it has not got to the stage
of finding that out before it panics? In other cases in the past non-deliverable
addresses in .forward files have just generated non-delivery responses. Any ideas
please?

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