[exim] Rewrite MAIL FROM: address

Need help!
We have Snap Appliance server sending out alerts using From: header like
hostname@ipaddress (eg. myserver@172.16.1.99).
It sent to our local smtp server using exim as MTA and relayed to our smart
host in our new ISP provider.
Now our new ISP rejected the email with the error:
2005-11-28 17:18:19 1EgrKF-0005ic-DI <= myserver@172.16.1.99 H= myserver
(myserver) [172.16.1.99] P=smtp S=503
2005-11-28 17:18:19 1EgrKF-0005ic-DI ** recipients@??? R=smarthost
T=remote_smtp: SMTP error from remote mailer after MAIL FROM:<
myserver@172.16.1.99> SIZE=1545: host smtp-relay [101.13.222.2]: 501 FROM
Invalid mail address, invalid domain name syntax

How do I rewrite MAIL FROM: address to a qualified domain address?
TIA.