How do I change the "mail from" for email alerts?

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

2009-06-04 12:26:17 Received from hobbit@ U=hobbit P=local S=929
2009-06-04 12:26:33 user-d5cf6b2f7d35@xymon.invalid <user-d5cf6b2f7d35@xymon.invalid> R=dnslookup T=remote_smtp: SMTP error from remote mailer after MAIL FROM:<hobbit@> SIZE=1973: host exchange.mydomain [10.10.10.21]: 501 5.5.4 Invalid Address
Process failed (1) when writing error message to hobbit@ (frozen)

I have reconfigured Exim4 to allow remote mail, but I can't find a way to make the email alerts from hobbit/xymon use a different "MAIL FROM" than "hobbit@", which our Exchange server knows is not a valid address.

I have tried making sure that the hostname is defined in the bbserver config and that the domain is configured in resolv.conf. I also tried creating an entry for "hobbit: user-5c8ec6c92842@xymon.invalid<mailto:user-5c8ec6c92842@xymon.invalid>" in /etc/email-addresses

I'm not a *nix guru by any means, and I'm sure this is something simple that I'm missing?