What purpose is served by using BASE64 beyond making the content able to
travel through a 7-bit-only hop? Since most (all?) hops are now 8-bit ones,
why not just send the actual 8-bit version of the data instead of converting
it to the 7-bit BASE64 format?
The answer to this question is quite complex. The 8BITMIME extension was
defined in 1994, and nearly all current MTAs can handle it. But all it
says is that the MTA can handle 8 bit characters. It doesn't affect the
rule that the message must consist of lines no more than 1000 characters
long with \r\n at the end of each. As far as I can tell, the main effect
is that people can send ISO-8859-x and UTF-8 without encoding, which is
useful but generally not a big deal.
BINARYMIME was defined in 2000 to avoid that issue, and invents a new BDAT
command that uses a byte count rather than escape sequence, so the message
body can be an arbitrary sequence of octets. Gmail, hotmail and
icloud/me.com support it, Yahoo and AOL don't, but I've never seen client
software that would take advantage of it.
What we've never seen is a quoted-unprintable encoding, which is like QP
but intended for binary data. It could be like QP without soft line
breaks and \r\n pairs are ignored. It's an obvious idea so there must be
some reason. Dave Crocker or John Klensin or Ned Freed would know.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail.
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