It can be a bit dense to read, but the "Content-Transfer-Encoding" section of RFC 1341 has all of the details: http://www.w3.org/Protocols/rfc1341/5_Content-Transfer-Encoding.html The situation kinda goes from bad to worse. Here's my summary: Background SMTP, by definition (RFC 821), limits mail to lines of 1000 characters of 7 bits each. That means that none of the bytes you send down the pipe ca