[Exim] string_sprintf expansion was longer than 8192

Situation: exim 4.10 with spamassassin

The paniclog is showing fairly regular incidences of reports like:

2002-09-29 20:13:47 17vjVi-0000pc-00 string_sprintf expansion was
longer than 8192

This seems to be a report from exim itself rather than SA, though
it seems to be somehow associated with invoking spamc/spamd (?)

The only relevant stuff to be seen in the mainlog is e.g

2002-09-29 20:13:46 17vjVi-0000pc-00 SA: SAEximRunCond expand returned: '1'
2002-09-29 20:13:46 17vjVi-0000pc-00 SA: check succeeded, running spamc
2002-09-29 20:13:47 17vjVi-0000pc-00 string_sprintf expansion was longer
than 8192

The mail body has been retained each time in e.g
/var/exim/spool/input/17vjVi-0000pc-00-D

and it can be seen to be a chain letter. Of course, we don't want it,
but we don't seem to be able to get rid of it by this means... The
sending MTA is evidently seeing us go away during its sending
attempts, so it's just retrying at intervals. This has been going on
for a fortnight now, at least.

I'm not sure how to get a handle on this. I've done various searches
for the symptoms but have come up blank so far (though I fear that
some helpful soul will now show me the answer in an all too obvious
place?). All the exim mailing list matches on this diagnostic are
old, and seem to have no relation to spamassassin.

thanks