Email Server and IP Address

In our organization, we have an email server behind a router with NAT enabled. In about two weeks’ time, we will have DSL installed, and a new IP address different from the current one in place. My question is, how and/or who should I contact to have email coming to our domain comes through the new IP address?
We currently have a third party hosting our web pages, only the email server is in house.
Thanks.