title: Is HTTP 501 appropriate for an unimplemented API?
body: <p>Is an HTTP 501 error appropriate for functionality which the server plans to support, but does not currently, such as a particular case of an API? For instance, if I was designing a webmail app and I couldn't yet delete emails with attachments, would it be appropriate to give 501 if I got a DELETE request on an email with an attachment?</p>
  
  <p>The RFC says that it should be used for an unknown method (e.g. a PARTY request), but it's not clear whether it should be used for other functionality as well.</p>