Welcome to the Community. I'll be happy to assist you with adding a PO number or Job name to your emails.
To do this, follow the steps below:
1. From the QuickBooks menu bar choose Edit, then select Preferences,
2. Choose Send Forms and then choose Company Preferences
3. Select the Appropriate Template and click Edit, then click the Insert Field
4. Choose Customer PO Number.
5. Make sure to click Save when you're done.
When the Invoice is transmitted via Email to the Customer their PO# will appear in the Subject Line of the Email to the right of the Invoice # (and other subject line content you have chosen to include). This means that Customers are going to immediately be able to more easily match the Email with attached Invoice to their own original Purchase Order saving them a great deal of time and effort. Give these steps a try and let me know how you make out. I'll be on standby in case you need further assistance.