For the first time in a decade, the US National Institute of Standards and Technology (NIST) has updated its secure email guide. The last effort of the NIST Agency in the development of email security guidelines is dated 2007. The new NIST guide is a document composed of 81 pages that aim to give recommendations and guidelines for enhancing trust in email. It also provides recommendations and guidance for email digital signatures and encryption (via S/MIME) The guide is out for public comment until May 1st, I suggest you to read it.”]
Source: http://securityaffairs.co/wordpress/45889/security/nist-guide-secure-email.html