We have a custom module for tracking Quote Requests/POs/Bills etc. We customised it so everyone could see everything in one place. 

The standard Purchase Orders module wasn't really suitable.

In the custom module, the standard email field won't connect to the Vendor Lookup field to allow auto-filling. I have a Vender Lookup and a related email, but when we send emails from it, the email won't auto-fill. I tried to set up a workflow to copy the email addresses but it seems workflows don't work with standard email fields. 

Is there a way around this?