Check receiver_email only if receiver_id was not checked.
In Paypal, this is possible to configure as receiver_email
a different email than the business email (the login email)
In Odoo, there is only one field for the Paypal email:
the business email. This isn't possible to set a receiver_email
different than the business email.
Therefore, if you want such a configuration in your Paypal,
you are then obliged to fill
the Merchant ID in the Paypal payment acquirer in Odoo,
so the check is performed on this variable instead of the receiver_email.
At least one of the two checks must be done, to avoid fraudsters.
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