[Exim] Process failed when writing error message

I have 2 mail servers running exim (v4.24) that receive about 50,000 to 100,000
emails daily. Recently I have noticed a minor problem with bounce message
processing. This happens on average to about 2 - 3 emails per week (not very
often considering the traffic the servers handle). I get the following error
sometimes:

2004-01-02 10:34:11 Received from john.smith@??? H=(yliao)
[192.168.100.226] P=smtp S=130772 id=000501c3d14d$fe73e140$f6957081@???
2004-01-02 10:34:12 huoxianhe2002@??? R=dnslookup T=remote_smtp: SMTP
error from remote mailer after end of data: host mx2.mail.yahoo.com
[64.156.215.6]: 554 delivery error: dd This user doesn't have a yahoo.com
account (huoxianhe2002@???) [-5] - mta227.mail.scd.yahoo.com
Process failed (1) when writing error message to john.smith@???
(frozen)Process failed (1) when writing error message to
john.smith@??? (frozen)Process failed (1) when writing error
message to john.smith@??? (frozen)Process failed (1) when writing
error message to john.smith@??? (frozen)Process failed (1) when
writing error message to john.smith@??? (frozen)Process failed (1)
when writing error message to john.smith@??? (frozen)Process failed
(1) when writing error message to john.smith@??? (frozen)
.........The "Process failed" error keeps repeating over and over and over ...

Here is what I have done to troubleshoot this so far:
1. Check obvious things like disk space and mem usage: these are far from being
a problem.

2. Checked the exim archives: Found one occurance of someone having a problem
with this. It was tracked down to someone doing a "killall exim" however, this
person did not get the repeating message like I am. Also none of the times are
related to any exim restart I might have done.

3. It seems that for some reason exim keeps tring to bounce the message but it
keeps failing for some reason. I have looked at the messages in question but
can not find anything obviously wrong with them.

I am not sure where to go from here.

Side note: there probably needs to be a "\n" after the error message as all
the Process failed lines end up on one line in the msglog.

Thanks,
Kenneth
University of Texas - Southwestern Medical Center