You can enable and customize behavior for return receipts for all accounts by going to:
Mac: Preferences > Advanced > General > Return Receipts
Windows: Tools > Options > Advanced > General > Return Receipts
Or you can customize the behavior for handling return receipts on a per account basis by going to:
Mac: Preferences > Accounts > [Your Account] > Return Receipts
Windows: Tools > Options > Accounts > [Your Account] > Return Receipts
Once this is configured, you can request a return receipt for a specific message you are sending by going to Options > Return Receipt from the Compose Window.
Note that it is up to the receiver to determine whether or not they wish to return the receipt.
Delivery Status Notifications
Postbox also supports Delivery Status Notifications (or DSN). From a compose window go to Options > Delivery Status Notification to add a DSN request to the outgoing message.