I'm unable to find what is causing this below (any insight please?)

From exim main log:

2002-09-20 19:30:43 17sXEO-0007Yz-00 ** [email protected] R=lookuphost T=remote_smtp: SMTP error from remote mailer after RCPT TO:<[email protected]>: host domain [x.x.x.x]: 550 cannot route to sender address <[email protected]>

The remote server is also Exim. I checked that host.server resolves to the correct IP address. May be this has to do with reverse lookup? I'm not sure.