Content Transfer Encoding

Internet User Agents, especially those subscribing to the Simple Mail Transfer Protocol (SMTP)— such as mail programs and message transfer agents (MTAs), impose a restriction of 7-bit US-ASCII characters and short line lengths on the transferred content. However, to be able to send a rich body of content, such as multipart attachments, non-English messages, and binary data including bitmaps and executables, it is necessary to have a standard that specifies an encoding scheme that allows client programs to work with rich content.

The Multipurpose Internet Mail Extensions (MIME) specification defines such a format for encoding and decoding complex message bodies. Base64 and Quoted-Printable are two of the encoding schemes defined by MIME that ensure that binary as well as non US-ASCII content will properly pass through all MTAs and can be interpreted by all MIME compliant user agents. Base64 is a compact and efficient encoding scheme for binary data, while quoted-printable is a generally human-readable scheme used for mostly text data. Line length in both cases is restricted to 76.