Redirecting users in the building to an internal IP instead of an external IP

i am no DNS expert, i am a novice at best. i have a question for anyone that knows how to run an internal DNS server.

our sales people have laptops and they come and go. they use DHCP which gives them DNS settings either from my server (while in the building) or whatever settings they need while on the road.

our desktops are set to check their email at 10.0.0.3 since they never leave the building. the laptops need to check email inside AND out of the building, so inside they would need to use 10.0.0.3 and outside they would need to use mail.domain.com which points to the external routable IP.

what i would like to do is create a record in our internal DNS server that says "if someone in the building wants to reach mail.domain.com, send them to 10.0.0.3 instead"

here is the tricky part. our internal domain is NOT the same as our public domain. we are known on the internet as mail.domain.com or ftp.domain.com, etc. but internally, the domain is cp.dom. (yes that is DOM not COM). so i can't just add a record named "mail" because it creates a record for "mail.cp.dom" instead of "mail.domain.com".

i know how to do this kind of change in a host file in the etc directory in win2k. i would just add a line like this:

10.0.0.3 (tab) mail.domain.com

but i need to do it on a network level so it applies to the laptops ONLY when they are in the building. when they leave, i need mail.domain.com to work as usual.

any suggestions?

thank you.