Derek,
Thank you for your reply.
From: Derek Atkins <warlord(_at_)MIT(_dot_)EDU>
Subject: Re: some requests
It's a SHOULD: you can be perfectly compliant and NOT implement it.
That's the point. A SHOULD is a strong recommendation to do something
that way, but it is not required to be implemented in order to be
compliant. Indeed, it's a directive to an implementor for how to
solve the problem in an interoperable way.
This explanation sounds that it is an optional. According to RFC 2119,
options should be specified as "MAY".
RFC 2119 also says that the full implications must be understood the
SHOULD functionality. So, for instance, if any Open/PGP UAs for
"qmail", which don't have the "From " problem, don't implement this
encoding stuff, we can't call them fully-conformant.
PS: How does applying a CTE break interoperability? Any MIME-aware
client MUST implement CTE anyways, so it should decode the base64 well
before it gets up to the user.
There are still many non-MIME UA users.
BTW, do you know any region where base64 is actually used for
text/plain? If we use base64, readability becomes drastically poor.
For example, Chinese people uses GB-2321 character set, which is 8bit
and base64 is suitable if required. However, they tend to send GB-2321
text with CTE: 8bit (without base64) for readability. (Most SMTP
channels in Asia are 8bit clean.)
--Kazu