Can a PO number automatically show in an email subject line to suppliers in the way an invoice number automatically does when sending a client an invoice?

Hi alan-create-this, thanks for joining the Community - there's currently no way to automatically include the PO number on the email subject line, this would need to edited manually at the point of sending. We'd welcome you to share any feedback that you have for this by clicking the Cog > Feedback in your account. 🙂