I have two domains on two separate networks. When email is addressed from
one to another, it is handled just like any other Internet email. The ISP
server has the DNS records for each domain and everything is working OK.

Until . . . the backhoe whacks the T1.

I would like to handle the DNS records to point to each of the domains so
email will be delivered through the still intact cabling between the
networks. Hopefully this will even speed up normal operation by avoiding
routing email traffic to the ISP for delivery back to the other domain.
My router is directly connected to both networks and T1. Both networks
are stub networks.

Each network has a DNS server to handle static hosts and dynamic dns.
That stuff is working just fine.

Suggestions are greatly appreciated.