When you create a purchase order, you have the ability to email the purchase order directly to your supplier. You can do this using the actions drop down menu and then "email purchase order".
This will generate a PDF and send it as an attachment.
If your supplier has an email address saved, Finale will automatically use their email address in the record to email it to. If you have multiple email addresses, this is how Finale prioritizes them. If the first is missing, it will use the second, then the third, and so fourth.