Simple internal IP address resolution for mail

I'm looking for an easy way to do the following.

- salesman with laptop accesses mail externally with Outlook Express using the 
company's external IP address, by using POP server defined as 
mail.theircompany.com - works fine outside the office

- when in the office (as there is no internal DNS server) this resolves to the 
external IP address, and can not connect

- all I want to do is have mail.theircompany.com resolve on the local LAN in 
the office to the internal IP address of the iSeries (192.168.99.99) instead of 
the external public IP address (that their ISP's DNS servers are returning)

Is there a simple way to do this using the 70-POP Server option in the DHCP 
server configuration (which IS running on their iSeries), OR do I have to 
configure a DNS server on the iSeries to resolve this ONE address 
(mail.theircompany.com) and have the present DNS server (which is actually the 
ISP's DSL router) continue to resolve all other address requests ?
 

Neil Palmer, Cambridge, Ontario, Canada
 
(This account not monitored for personal mail,
remove the last two letters before @ for that)


-l.