Receipt emails with empty 'Reply-To:' headers

If attempt to send a back-office contribution receipt (eg. at a path like 'civicrm/contact/view/contribution?reset=1&action=update&id=720&cid=913&context=contribution' and check 'Send receipt?') the resulting email has an empty 'Reply-To:' header field.

Some mailservers are configured reject emails with invalid Reply-To headers, eg.:

host xxx.xxx[xxx.xxx.xxx.xxx] said: 554 5.7.1 This message has been blocked because the return email domain is invalid.(domain name has invalid syntax)

I'm not sure whether a rejection of emails like this is strictly to spec, but rather than trying to get mailservers around the world to reconfigure their servers, it would be easier to omit the Reply-To: header if it is empty.