• HTTP (HyperText Transfer Protocol) is the standard encoding for web pages. Similarly, Internet mail uses SMTP (Simple Mail Transfer Protocol) for encoding and sending electronic mail. As the name implies, the protocol is unsophisticated. It employs only simple 7-bit ASCII (American Standard Code for Information Interchange) text. In order to send e-mails with content that doesn't fit in ASCII, such as multimedia or languages other than English, the content must be converted, or encoded, into ASCII. The three standard encoding types are identity, base64 and quoted-printable. Their collective label is Multipurpose Internet Mail Extensions, or MIME.
  
  

Identity Encoding

• Another name for identity encoding is binary encoding, due to the format of the result of the transformation. When a mail client encodes non-ASCII content using identity, the result is a series of zeros and ones--binary. Although the official document governing MIME defines identity encoding, according to the same document, SMTP currently does not use the mechanism.
  
  

Base64 Encoding

• Base64 encoding's other name is 8-bit. Base64 encoding groups data into eight 8-bit-long chunks, which it further divides into 6-bit-long chunks, then translates four of these at a time into a single digit in the base64 alphabet. It is an efficient process, with encoded data only 33 percent larger than the original unencoded data, but it's impossible to understanding just by looking at it. Mail clients generally use base64 encoding to send multimedia content.
  
  

Quoted-Printable Encoding

• Quoted-printable encoding also goes by the alias 7-bit, because it is the default method for converting content to the 7-bit form SMTP needs. It is similar to base64 encoding in that it encodes by processing 8-bit "octets" of data at a time. The difference is that the content, once 7-bit encoding gets through with it, is largely readable text. Mail clients commonly use quoted-printable encoding when sending non-English characters, which computers encode using 8 bits.