Purchase Order Email From field through API

I have 2 questions:

1) How do I specify the "from" email in Email Purchase Order API?

2) What does this error message "The email address for this customer was not found in the customer's details. Hence the mail could not be sent." mean?

I get this error when I leave the json input blank in the Email PO API.

My contact is a vendor contact type and the record also has contact persons with email information.

Thanks,

Thanh