A few users who pay by check need to have receipts emailed upon posting of payment. This is almost impossible manually. How about a toggle to do it automatically?
It's not possible to do with cash/check (there might be cases where you wouldn't want to do it, like a correction or something). But you can set it to override on a per customer basis for auto-processed ones (under the customer's Payments screen).
Select "Email" in the Override Payment Receipt Type field on the Payments screen and make sure that there is a value in the Override Email field on the Customer Info screen. When payment is posted the receipt is emailed.