Mapping MX servers of one domain onto another domain.

[Manjunath H N]

Hi,
 
I have registered a domain example.com, now I have a client requirement,
the requirement is as follows.
 
The client has a registered domain client.com, they have a MX record
pointing to 3 servers, now they want a requirement, to map their MX
records in client.com domain to our domain example.com.
 
How can this be achieved ???? As example.com is not a authorized server
to host MX records for client.com.
 
This is the requirement
 
Existing:-
 
client.com (MX records)
client3.com     internet address = a.b.c.d
client2.com     internet address = e.f.g.h
client1.com     internet address = i.j.k.l
 
Requirement:-
 
To map the MX of example.com to client.com
 
TIA
Manjunath H N