[Exim] exim failing to find users

Exim 4.12:
Over recent weeks I have had a few instances of exim loging to the
paniclog because it can't find the exim user or root! The messages are either

Exim configuration error in line 48: user root was not found
or
Exim configuration error in line 80: user exim was not found
or
Exim configuration error in line 499: group mail was not found

Line 48 is "never_users = root"
line 80 is "system_filter_user = exim"
line 499 is "group = mail" in the local_delivery transport.

These events have all happend at arount the same time of day (11:56, 11:57, 11:49
and 11:51) with each resulting in several paniclog messages over a few seconds. I
first suspected some system activity, but I would exepect the times to be closer if
cron were doing something.

At the same times as the paniclog messages exim has rejected e-mail because the RCPT
TO recipient is unknown (though the recipient actually does exist). I deduce that
exim is calling getpwnam and so on and getting a failure response to everything at
these times.

The system uses NIS for the password and group files. Another machine is the master
with the mail server being its own slave server (ypwhich shows it to be latched on
to itself every time I check). The make processes on the master server do not
co-incide with these events. I have checked the yp logs and see no NIS updates
taking place at the relevant times.

Clearly, I only notice that this has happened some time later when I see the
paniclog.

I am at a loss as to how to investigate further. Does anyone have any ideas as to
how I might work out what is going on and what I might do to generate some
diagnostics when it happens next?

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