If an email address is blocked or if the contact method gets stuck so it does not include email, this will prevent the parent from receiving any emails or making any payments as School Gateway requires an email address to send a payment receipt to.
The contact method usually updates once the email address is re-enabled. However if the contact method doesn't update as expected or there is no option to re-enable, if you opt the parent out and then opt them back in again via Dashboard>Linked People this should force the contact method to update to include email.