I updated to 4.6.27, and recurring donations started re-sending out email receipts each month. The old behaviour was to send an email receipt when they made their contribution but not a monthly email. I'm using the Stripe payment processor.
We do not use Stripe so this might not help. But this is how it works for iATS.
Go to Administer > CiviContribute > iATS Payments Settings (the very bottom option) One of the check boxes is "Enable email receipting for each recurring contribution" If this is checked you will be sending a receipt with each donation.
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1While the specifics of this response don't apply more general "check your payment processor settings" was the issue. Turns out this setting for the stripe payment processor can't be set in a form and must be modified in code. – DaveBagler Apr 6 '17 at 15:42