There are many ways to do this; some of them could have been done even "outside the box".
It might be configured using /etc/mail/mailertable. (If it's done this way, then the previous admin probably did not know about the DS setting of sendmail.)
Or your server might have iptables redirection rules to force all outgoing connections to port TCP/25 to the relay server.
Run:
iptables -L -v -t nat
If you see rules with target "DNAT" and destination is something like "anywhere tcp dpt:smtp --to-destination
", then mail is probably redirected this way.
The cause might even be external to your server: your site's DNS server might be configured to respond to all requests for remote domains' MX records with the address of the relay server.
Test using "dig" and the domain name of a non-local email recipient. For example, if the recipient was "someone@nonlocal.com.example", you might run:
dig nonlocal.com.example mx
If this returns records pointing to your relay node, then the DNS server is causing the redirection.
Another way to enforce such a redirection externally would be to use transparent proxying technology in your site's outgoing router.
MK
MK
