[exim] How do I force a certain "MAIL FROM" address?

My /var/spool/exim4/msglog is filling up with messages similar to:

2009-06-04 16:02:30 it@??? <IT@???> R=dnslookup T=remote_smtp: SMTP error from remote mailer after MAIL FROM:<hobbit@> SIZE=1773: host hhcgvl-msbs01.hhc.int [10.10.10.21]: 501 5.5.4 Invalid Address
Process failed (1) when writing error message to hobbit@ (frozen)

It appears to me that the problem is with the "MAIL FROM" address.

I have tried modifying my /etc/email-addresses with:

hobbit: monitor@???
root: hballinger@???
hobbit@: monitor@???
hobbit@localhost: monitor@???

I have also tried modifying /etc/exim4/exim4.conf.conf with:

begin rewrite
hobbit@ monitor@??? s
Hobbit@ monitor@??? s
hobbit monitor@??? F
Hobbit monitor@??? F

I have not been able to find a way to force the from address to be monitor@???

Any ideas?