What is the difference between RFC 5321 and RFC 5322? What are the SMTP and MIME standards? What is the difference between a MIME content type and a MIME transfer encoding? Briefly explain base64 encoding. Why is base64 conversion useful for an e-mail application? What is S/MIME? What are the four principal services provided by S/MIME? What is the utility of a detached signature? What is DKIM?