Centos 5.3.

Some application or a conf file must be misconfigured somewhere. Our maillog file consistently lists entries where email is attempting relaying through and old mail relay host. Have looked in all the usual places (sendmail.mc, aliases, hosts, httpd conf) and for the life of me can't find where this is coming from (although the source seems to be Apache but that old host is no where in the httpd configuration file). Any ideas as to how I can find the rogue attempts to relay to this old host?

Many thanks for your help!