When you want to add a new contact for a supplier so they can be automatilied email when an order is placed.

Go to Purchasing and then Suppliers. 

Click on the 3 dots next to the Supplier you want to add the email and then Contacts

Click on the Plus on the right hand side of the grid to add a new contact.

Select an existing contact or add a new one by clicking the plus on the grid. 

If you want this contact email select the "To Be Emailed?" check