US-ASCII text
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... receiving User Agent can also assume
that plain US-ASCII text was the sender's intent. Plain US-ASCII
text may still be assumed in the absence of a MIME-Version ...
... that plain US-ASCII text was the sender's intent. Plain US-ASCII
text may still be assumed in the absence of a MIME-Version or the
presence of an syntactically invalid Content-Type ...
... the resulting octets are unlikely to be modified by mail transport.
If the data being encoded are mostly US-ASCII text, the encoded form
of the data remains largely recognizable by humans. A body which is
entirely US-ASCII ...
... character of a CRLF pair is illegal. This case can be
the result of US-ASCII text having been included in a
quoted-printable part of a message without itself
...