ReturnPath is what shows when the email fails to send. IT should be set to an actual email address like: firstname.lastname@example.org.
Setting the return path to the value from the To address makes no sense, since the email is sent to the return path when the To address is invalid.
This just informs you when an email is not valid so you can remove it from your list. You can probably just remove that line entirely.
That might actually be the cause of the issue you are having.