Internal Mail DNS questions

[AVB]

I am having trouble sending mail internally from Server2 to Server1.
Server2 sends mail fine to any other domain but just not internally.
Server1 is our main Mail server and Server2 just sends message from our
database application. Both servers are on the same W2K subnet and
domain and I have one DNS server only for the internal machines. Our
internal domain name is different than our Mail Domain name too so I'm
not quite sure how MAIL DNS Queries operate in this regard.

I set up both domain names in our internal DNS server with a MX and A
record for the main mail server (server1) in the external domain and
when I run

Nslookup -q=mx server1.externaldomain.com I get:
server1.externaldomain.com  192.x.x.x (internal IP)

if I run NSlookup -q=mx externaldomain.com I get:
responsible mail addr = admin.internaldomain.com

if I delete the external domain in our internal dns server and run
nslookup -q=mx server1.externaldomain.com I get the correct external IP

and if I run nslookup -q=mx  externaldomain.com I get the correct
server1.externaldomain.com and IP

Not sure how to succesfully set up the DNS server for this. Anyone have
any thoughts?  TIA!