When you perform email notifications for a Purchase Order, by default the Reply-To in the email will be the current user; the email address of the user performing the email delivery request.
Here is how you can specify a custom Reply-To email to be used for all PO email notifications
1. Navigate to your Administrative section
2. Locate the Outbound Emails section and click Add in the top-right
3. Create a new entry and choose Key = purchaseorders.deliver
4. In the Reply-To field enter a single email address of your choice
5. Save
When you request an email notification you should see this email address specified on the screen.