Return path is optional, it is the address that the email will be sent to if the recipient email does not exist.
think of it this way:
On a regular snail mail letter, you add a return address on the outside of the envelope, this is used by the post office if the letter cant be delivered. this is the equivalent of the Return Path.
Inside the letter, you may instructions for replying to a different address. This is the Equivalent of the Reply To
The Return Path should use the From Address if it is left blank, though some servers dont follow the rules.
Try leaving it blank, then inspect the headers for a message, and see what is used, it should use the from address.
