Why aren't donation receipts being emailed to customer?
This seems to be a known, critical, and unresolved bug with WordPress and PayPal since 4.5.4. It has been reported as issue 16181, Email reciepts for contributions being marked as sent but are not sending, and therein is noted as related or a duplicate to 15629, Paypal IPN not working on Wordpress the first time, which is also referenced in the other Civi.SE question.

Kurund suggests testing patch 5647 to see if it resolves the problem.



Update: Both Issue 15629 and Issue 16181 are marked as resolved as of 4.6.3.