First of all, the terms exchanged back in forth via email are usually subject to investor approval. Also, there are probably written guidelines in place that require that any short sale purchase terms be formally documented in a letter from the lender.
Review the purchase contract and any addenda applicable to your transaction. Also look closely at all of the short sale documents and policies provided by the servicing company or lender. Any of these documents may contain information that addresses the issue of whether the content of emails can be construed as binding on either party.
These documents may contain language clearly indicating that there is no binding obligation unless and until the lender/servicing company issues an official Agreement Notice or Approval Letter.
Contact a local real estate attorney experienced in short sales for help if you are currently trying to short sale your property.