Weird "No Such User Here" Error

I'm trying to send mail from mysite.com (my own server), to a remote server that hosts user@remotesite.org.

From server1.myhost.com as root I ran:

Code:
exim -bt user@remotesite.org
user@remotesite.org is undeliverable: No Such User Here
From server2.myhost.com (that is located at the same rack as server1):

Code:
exim -bt user@remotesite.org
user@remotesite.org
router = lookuphost, transport = remote_smtp
host lobosuelto.arnet.com.ar [200.45.191.210] MX=5
host corderoatado.arnet.com.ar [200.45.191.211] MX=5
From server3.myhost.com (another server on another different rack):

Code:
exim -bt user@remotesite.org
user@remotesite.org
router = lookuphost, transport = remote_smtp
host lobosuelto.arnet.com.ar [200.45.191.210] MX=5
host corderoatado.arnet.com.ar [200.45.191.211] MX=5
On the three servers I'm using the exact same Exim configuration, the same name servers at /etc/resolv.conf, same cPanel Release version, but I still don't understand why at server1.myhost.comI got the error. The only difference is that at server1.myhost.comI'm running CentOS 5, instead of RHE 5.2 (server #2 and #3).

Already tried disabling Sender Verification/Callouts at Exim Config, resetting exim config to default but the error stills.