rfc1939.txt POP3 rfc2821.txt SMTP rfc2822.txt Internet Message Format rfc977.txt NNTP rfc2045.txt MIME 1 rfc2046.txt MIME 2 rfc2047.txt MIME 3 rfc2048.txt MIME 4 rfc2049.txt MIME 5 rfc2060.txt IMAP4 rfc2015.txt MIME Security with Pretty Good Privacy (PGP) rfc2487.txt SMTP Service Extension for Secure SMTP over TLS