Content-Transfer-Encoding: 7bit

The 7bit is the most fundamental message encoding. Actually, 7bit is not encoded; 7bit encoded files are files that use only 7-bit characters and have lines no longer than 1000 characters. CR (carriage return) and LF (line feed) characters can only occur as pairs to limit the line length to 1000 characters, including the CRLF pair. NULL characters are not allowed. 7bit encoded files need no encoding or decoding. This is the default.