The Content-Transfer-Encoding header specifies the encoding of the associated message body so that it meets SMTP requirements. SMTP requires data for transport to be in 7-bit US-ASCII characters with lines no longer than 1000 characters.
Content-Transfer-Encoding values are described in detail in RFC 2045 Section 6, available at tp://www.ietf.org/.
Encodes stream-based data. See RFC 2406 Section 6.8.
Encodes data that consists of printable characters in the US-ASCII character set. See RFC 2406 Section 6.7.
Used for data that is not encoded. The data is in 7-bit US-ASCII characters with a total line length of no longer than 1000 characters. See RFC2406 Section 2.7.
Indicates that the transfer encoding is unknown.