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agreement
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... interoperability
with little potential benefit. Their use is allowed only as the
result of an agreement between cooperating user agents.
...
... beginning with "X-" is explicitly reserved to the IANA for future
use. Private agreements about content-transfer-encodings are also
explicitly discouraged.
...
... position where the column number is a multiple of 8 (counting the
first column as column 0)." Apart from this, any use of the control
characters or DEL in a body must be part of a private agreement
between the sender and recipient. Such private agreements ...
... agreement
between the sender and recipient. Such private agreements are
discouraged and should be replaced by the other capabilities of this
document.
...
... registration with IANA,
or by private agreement, in which case the character set name must
begin with "X-".
...
... A Content-Type value beginning with the characters "X-" is a private
value, to be used by consenting mail systems by mutual agreement.
Any format without a rigorous and public definition must be named
...