MX and Split DNS

[shahs1]

Hi,

Our company is splitting the DNS servers (internal and external). Our email
server is internal (inside the firewall). It was on the external DNS server.
We do have an entry for emails erver on our inside DNS server.

The problem is, when we remove the MX record for our email server and point
it to our firewall (of course before doing that we already make sure that
our firewall know where to deliver all the emails, also we have it on our
internal DNS server), email stop working. When we checked the log, it say
that email was sent by the firewall and received by the firewall. When it
worked, email was sent by the firewall and received by email server.

Before remove (working):

                 IN      NS      ns1-ext.example.com.

                 IN      MX 5    mx01.example.com.
                 IN      MX 10   mx02.example.com.

mailhost     IN      A       111.222.333.444
mx01          IN     A        111.222.333.444
mx02          IN     A       999.888.777.666

After remove (not working):

                 IN      NS      ns1-ext.example.com

                 IN      MX 10   mx02.example.com.

mx02          IN     A       999.888.777.666


Does anybody know what am I doing wrong?

Thanks.
Ridhuan.