Wenn es sich um eine Person handelt: In future kindly turn Mr. / Ms x who will be your future contact person.

As I will no longer be with the company as from XXX I would ask you to use the following contact address for the Purchasing Department in future.
or
As I will be leaving the company on XXXX, please contact Ms/Mr CGJKJ in the Purchasing Department who will assist you with any queries you may have in the future
(IMO it is better to give a contact person rather than a contact address.)
Or about a million variations thereof I guess