Testcase to check if the bpartner contact and the billto contact email are set correctly.
Open window bpartner, search for G000Y
Make sure User U is set as billto contact for purchase orders, User Z is set as default user, and both have email addresses
Also make sure that the User_ID of U is lower than the ID of Z (if not, create a new user as default user; the default one should have the higher / highest ID)
Open window product, search for P0001, open tab bpartner
Make sure that P0001 is set for G000X as customer, and G000Y as the current vendor
Create a sales order for G000X, with P0001 in orderline
Gear, select Bestellung aus Auftrag generieren, OK
In the purchase order, delete User U as billto contact, save