Hehe, I’ve never been much on IRC or similar channels. It’s just too “noisy” for me, and I’ve had little time lately anyway.
Let me be more concrete. Each global DNS entry for MX will NOT contain an IP address but a domain name, e.g.
nic.cz. 1800 IN MX 10 mail.nic.cz.
I would personally just redirect that name mail.nic.cz to a different address on LAN, and you can do that relatively simply. There might be some clickable way, even, I’m not sure, but it should certainly still work to follow DNS redirection