Forward Lookup Zone between two exchange servers

I have Two exchange servers on the same network, both going through the same firewall and broadband connection. They both have different static IP addresses, which are port mapped on the firewall though to the corresponding server. The servers are on different subnets and are in a Business centre environment. Both servers can send and receive fine but not from and to each other.

EX1 - which hold domain abc.com
EX2 - which holds domain 123.com

What I want to do is set it up so that when user on EX2 send to abc.com it goes directly to the EX1 through the LAN. I think this can be achieved by using a DNS Forward lookup Zone. Please correct me if I am wrong.

So this is what I did on EX2:
I created a New Primary Zone and called it abc.com. In here a created an "A" host record for mail pointing to its LAN IP. And now people inside the network can go to mail.abc.com and it goes to its LAN IP address, checked this using nslookup.

I then created a MX record in the same zone for mail, with an MX priority of 10. I did this because the MX tags for abc.com are point to mail.abc.com.  This is where I think I am stuck. I have done an nslookup, set type=mx abc.com on EX2 and I get the following results:

Primary Server name = EX2.123.local
Responsible mail addr = hostmaster.123.local
Serial = 5
Refresh = 900
Retry = 600
Expire = 86400
Default TTL = 3600

It looks like its pointing at its self, for the other server MX tags?

Any help would be appreciated.

I am missing any config on the other server? Do I need a reverse Lookup Zone? Confused !